This vegetarian version of Croque Monsieur starts with thick, crusty slices of white sandwich bread. I stuff the inside with chargrilled slices of zucchini made earthy with herbes de Provence and tons of Emmenthaler, which gives a more delicate flavor than traditional Gruyère. Over the top goes a béchamel flavored with piquant Dijon mustard and a scrape of nutmeg. Then, more cheese, set under the broiler to bubble and blister. It's everything you want in a Croque Monsieur, made just slightly more delicate—hence, Croque Mademoiselle.
- 1 medium zucchini, cut into 4 long planks
- 2 teaspoons olive oil
- Kosher salt and freshly ground black pepper
- Pinch of herbes de Provence
- 1/2 tablespoon unsalted butter, room temperature, plus more for spreading on the bread
- 1/2 tablespoon all-purpose flour
- 1/2 cup milk
- 1/2 tablespoon Dijon mustard
- Pinch of freshly grated nutmeg
- 2 (2/3-inch) slices of fresh white sandwich bread
- 1 1/4 cups grated Emmenthaler cheese
Preheat a grill or grill pan to high heat. Toss the zucchini lightly with olive oil, salt, pepper, and herbes de Provence. Grill until charred and cooked through, about 4 minutes per side. Set aside.
In a small saucepan, melt the butter. Whisk in the flour and cook over low heat for 1 minute. Whisk in the milk, and continue whisking until the mixture has thickened enough to thickly coat a spoon. Set aside.
Preheat broiler to high. Spread one side of each slice of bread with butter. Place 6 tablespoons grated cheese on un-buttered side of one slice of bread. Top with the zucchini, then top with 6 more tablespoons cheese. Close sandwich, buttered side out. Toast the sandwich in a nonstick skillet over medium heat until golden brown on both sides, about 3 minutes per side.
Place the sandwich on a foil-lined rimmed baking sheet. Spread the top with the white sauce, then top with remaining 1/2 cup cheese. Broil until the cheese is toasted. Serve immediately.