Prep time: 25 minutes
Cook time: 12 minutes
Recipe by Rachael Ray from "The Book of Burger" (Atria Books).
For the spinach pesto
2 packed cups baby spinach
1/4 cup walnut pieces, toasted
1/4 cup grated Pecorino Romano or Parmigiano-Reggiano cheese
1 clove garlic, grated or pasted
A few grates of nutmeg
Kosher salt and pepper
1/4 to 1/3 cup extra-virgin olive oil
For the portobello caps
4 large portobello mushroom caps
1/4 cup extra-virgin olive oil
2 tablespoons Worcestershire sauce
2 tablespoons balsamic vinegar
2 tablespoons finely chopped rosemary
1/2 pound fresh mozzarella, thinly sliced
4 crusty rolls, split
Baby spinach leaves
Thinly sliced red onions
Make the spinach pesto: Place the spinach in a food processor with the walnuts, cheese, garlic and nutmeg; season with salt and pepper. Turn on the processor and stream in enough olive oil to form a thick pesto.
Make the portobellos: Brush the portobello caps with a damp towel to clean them. In a large plastic food storage bag, combine the olive oil, Worcestershire sauce, balsamic vinegar and rosemary. Add the mushroom caps and slosh around to coat with seasonings.
Heat a grill pan or large skillet over medium-high heat. Shake the marinade off the mushrooms and cook the caps, turning once, 10 to 12 minutes, or until well browned on both sides. Season with salt and pepper. Top the caps with the mozzarella, remove the pan from the heat, and tent loosely with aluminum foil. Let stand 1 to 2 minutes to melt cheese.
Place the mushrooms on the roll bottoms and top with pesto, a mound of baby spinach, and a few slices of red onion. Set the roll tops in place.