Portobello mushrooms are considered the vegetarian stand-in for meat on a burger, even if all the two have in common is being brown, round, and grilled. In this cheesy, double-decker sandwich, a slathering of pesto and a crunchy ciabatta bun add extra flavor and texture, for a truly tasty veggie burger.
10 ounces Monterey Jack or provolone cheese, thinly sliced
4 to 8romaine lettuce leaves
Place the vinegars, garlic, measured salt, and measured pepper in a medium, nonreactive bowl. While whisking continuously, add the oil in a slow, steady stream until evenly incorporated.
Place the mushrooms cap-side down in a single layer on a baking sheet and pour the marinade over them. Turn several times to coat, and let sit at room temperature for 30 minutes.
Heat a grill pan or outdoor grill to high (about 450°F to 550°F). Slice the ciabatta widthwise into 4 pieces, each slightly smaller than the width of the mushrooms (you may have some bread leftover). Slice each piece in half horizontally to make 4 tops and 4 bottoms. Place the halves cut-side down on the grill and cook until brown and toasted, about 2 to 3 minutes. Remove the bread to a cutting board or work surface.
Spread 1 heaping teaspoon of the pesto on each ciabatta top and bottom (2 teaspoons per sandwich—you’ll probably have some pesto leftover). Place 2 tomato slices on each bottom and season with salt and pepper. Set aside while you grill the mushrooms.
Place the mushrooms cap-side up on the grill, close the grill, and cook until just starting to become tender, about 4 minutes. Flip the mushrooms, top each with a quarter of the cheese, close the grill, and continue cooking until the mushrooms just give way when pierced with a knife and the cheese is melted, about 4 minutes more.
Remove the mushrooms from the grill. Stack 2 mushrooms on top of each other, cheese side up, and place on a bottom piece of bread. Repeat with the remaining mushrooms and bread bottoms. Top with the lettuce leaves and bread tops and serve immediately.