A gratin of summer vegetables should be the easiest thing in the world to make in August: a no-recipe breeze for the carefree summer cook. Slice up squash, tomatoes, etc.; arrange attractively; bake.
But this sunny, soap-commercial method has never worked for me. My gratins are often swamped with tomato water, filled with slices of raw onions and overcooked zucchini, and topped with a pale, dusty desert of bread crumbs.
This summer, I set out to solve all of the problems, feeling my way to a not-soggy, crisp-topped, satisfying gratin.
When zucchini and tomatoes are plentiful (read: overwhelming), it is tempting to pile lots and lots of them into a gratin. But part of what makes these summer vegetables fresh and delicious is that they are full of water. That water is the enemy of your gratin, and must be approached ruthlessly. (Winter vegetables, like potatoes, pumpkins and parsnips, have less water and more starch, and make beautiful gratins.)
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The simplest way to get rid of water is to apply heat. And so it’s a good idea to cook some or all of the ingredients before they ever see the inside of the baking dish. Cooking is the most efficient way to evaporate all that water in vegetables, and has a side benefit of concentrating their flavors.
Fortunately, the cooking can happen any way you like. You can sauté or broil or grill the vegetables. In fact, you can cook some of them and leave others raw; for example, as in the recipe here, if you cook the tomatoes, you can leave the squash alone. Since the casserole is baked uncovered and at a high temperature, water will also evaporate in the oven.
Here are some universal tips for achieving a crusty, caramelized gratin:
▪ Do not crowd the pan. It is tempting to jumble 3 pounds of raw zucchini and tomatoes into a baking dish, but that way lies sogginess. The vegetables should overlap, but not as closely as fish scales. They should look more like roof shingles, with plenty of exposure to air. If you are determined to bake a large quantity, use two pans, or cook all the vegetables first.
▪ A baking dish is pretty, but a heavy heat-hugging skillet is better at cooking the vegetables from below.
▪ Any uncooked ingredients should be slicked with oil before arriving in the oven. Oil conducts heat and will prevent the slices from sticking together, allowing hot air to circulate.
▪ Never cover a gratin containing raw vegetables while baking. This creates a steam bath inside the dish.
Provence is the natural habitat of the summer vegetable gratin (or, as it is called there, a tian), so for further improvements I went to the source. In her classic 1976 cookbook “The Cuisine of the Sun,” about the cooking of the French Riviera, the author (and Nice native) Mireille Johnston wrote that tomatoes that are to be baked should be cooked before going into the oven.
Fry them gently until “all their excess water is cooked away and they look transparent – like candied fruit,” she wrote. “In Provence, they say that they must look like a vitrail – a stained-glass window.” And indeed, they do. (A more timesaving way to incorporate tomato flavor in a gratin is to use a thick tomato sauce to coat the bottom of the pan.)
Traditional Provençal tians are topped not with cheese but with bread crumbs. But bread crumbs are not always the dry, microscopic shards familiar to us from cardboard tubes. “They are a terribly misunderstood ingredient,” said Jane Sigal, a writer who worked in Provence in the 1980s as a kitchen assistant to Julia Child’s co-author Simone Beck.
Although the distinction has mostly been lost in traditional cooking, she said, there are two distinct kinds of bread crumbs: dry and soft.
Dry bread crumbs are the familiar sandy style that are made from stale bread, and they are good for coating food that’s to be fried. (Panko crumbs, made from Japanese-style white bread, are even better.)
But fresh bread crumbs are semisoft bits of fluff, ranging in size from a kernel of popcorn to a fully popped piece. They have taste and a bit of chew, are made from bread that is no more than one day old, and are precisely what you want for a bread crumb topping. Dry bread crumbs will never bake into a real crust, but fresh bread crumbs will – as long as richer ingredients are added with a generous hand.
“If there’s one thing people can do to improve their crusts, it’s to add olive oil,” Sigal said. The bread crumbs should be thoroughly moist, with an oily sheen. Cheese is optional but helps the crust form. For extra frills and flavors, you might add minced garlic and parsley to the bread crumbs, producing the classic Provençal topping for baked tomatoes.
What with all the slicing and precooking, making a gratin – let’s be honest – can demand quite a lot of time (although the skill set required is entirely basic). To justify the effort, I made this recipe substantial enough to serve as a vegetarian main course or to complete a meal alongside a simple, grilled protein like a steak or a coil of Italian sausage. I added a base layer of sautéed alliums – onions, shallots, garlic, leeks are all fine – to make it savory. A top layer of fresh bread crumbs, cheese and lashings of olive oil bakes into the crispy crust of dreams.
To make it even richer, you might insert slices of mozzarella or add scoops of ricotta.
However, it’s also fine to lighten and simplify: Do just two of the layers, or even just the zucchini and squash. The above rules still apply, and will still make a deeply pleasing gratin.
Summer vegetable gratin
Total time: 1 1/2 hours
Serves 8 to 12
For the base layer:
2 medium onions, thinly sliced
1/4 cup olive oil
2 red bell peppers, thinly sliced, or 2 additional onions
1/2 teaspoon red pepper flakes
4 cloves garlic, smashed
For the optional tomato-bread crumb topping:
1 1/2 pounds plum or other ripe tomatoes
1/4 cup olive oil
1 cup shredded Parmesan or Gruyère cheese
For the gratin:
1/4 cup olive oil, more for baking
1 1/2 pounds zucchini, sliced 1/4-inch thick
1 1/2 pounds yellow squash, sliced 1/4-inch thick
1/4 cup freshly chopped basil or parsley, more for garnish
Salt and black pepper
Make the base layer: In a large, heavy ovenproof skillet or enameled cast-iron pan (10 to 12 inches across), combine onions and olive oil and heat to a sizzle, stirring to separate. Add bell peppers, red pepper flakes and garlic. Cook, stirring, over low heat until peppers are very soft and onions are browned, about 20 minutes. Remove from heat and remove garlic, leaving remaining mixture in the pan.
Meanwhile, make the topping (if using): Core tomatoes and slice them 1/4-inch thick. Lay on paper towels to drain for 10 minutes. Heat 2 tablespoons oil in a large skillet over low heat. Add tomatoes and cook very slowly, turning once or twice, until liquid has bubbled away and flesh is cooked through, about 8 minutes. (Do not overcook, or tomatoes will fall apart.) Turn off heat and let slices cool in skillet; they will continue to dry out. Tear baguette into pieces and pulse in a food processor to make coarse, fluffy, pea-size crumbs. Add remaining 2 tablespoons oil and the cheese and pulse to combine.
Assemble the gratin: Heat oven to 425 degrees. (If your oven has a convection feature, use it, reducing baking temperature to 400 degrees.) In a large bowl, combine oil, zucchini and squash, and toss well until lightly and evenly coated. Add basil, sprinkle generously with salt and pepper, and toss again.
On top of the base layer in pan, arrange squash and zucchini slices around the inner rim of the pan, standing on their edges in roughly alternating colors. Pat down into the pan so slices overlap and lie down, like shingles or fallen dominoes. Repeat to make another circle inside the first, and again if necessary, until pan is filled. Sprinkle with salt and pepper.
Brush oil over the top of the gratin and transfer to oven. Bake 30 minutes. Raise oven temperature to 450 degrees (425 degrees for convection), or heat the broiler.
If not using topping, brush surface again with oil. If using topping, arrange tomato slices in one layer on top of the par-baked gratin. Spread bread-cheese mixture over tomatoes and press down gently.
Bake or broil until vegetables are browned around the edges or crust is crisp and golden. Let cool slightly and serve hot or at warm room temperature. Garnish each serving with herbs.