This fully loaded vegetarian lasagna has two generous layers of sautéed mushrooms and spinach between creamy ricotta, tomato sauce and noodles and is great for a crowd. Grated fontina cheese is mixed with mild mozzarella for a little sharpness and nuttiness that highlights the mushrooms.
Preheat the oven to 400˚ F. Bring a large pot of salted water to a boil. Add the noodles, stirring to keep them from sticking; cook until pliable but al dente, about 6 minutes. Drain and rinse under cold water. Separate the noodles with your fingers; set aside.
Heat a large skillet over medium-high heat; add 2 tablespoons olive oil. Scatter in the mushrooms and cook, undisturbed, until browned on the bottom, 1 to 2 minutes. Stir, then continue to cook, stirring occasionally, until the mushrooms are well browned and no liquid remains in the pan, about 5 more minutes. Add the remaining 1 tablespoon olive oil; stir in the spinach, garlic and thyme and cook until heated through, about 1 minute. Season with 1/2 teaspoon salt and a few grinds of pepper. Set aside to cool slightly, breaking up any clumps of spinach with a wooden spoon or a fork.
Mix the ricotta, 1/2 cup Parmesan, the egg and parsley in a medium bowl. Season with 1/2 teaspoon salt and a few grinds of pepper.
To assemble, spread 1 cup marinara sauce in a 9-by-13-inch baking dish. Arrange 8 noodles in the dish perpendicular to the long sides so that 4 noodles hang over one long side and 4 hang over the other long side; the noodles should meet in the center of the dish. You'll use the overhanging noodles to create the top layer.
Spread half of the ricotta mixture over the noodles in an even layer. Top with half of the spinach mixture. Sprinkle with one-third each of the mozzarella and Fontina. Spread 1 1/2 cups marinara sauce on top, then arrange the remaining 4 noodles parallel to a long side over the sauce. Press down slightly to compress the layers. Spread the remaining ricotta mixture over the noodles and top with the remaining spinach mixture. Sprinkle with another third of the mozzarella and Fontina. Spread with 1 cup sauce. Fold the overhanging noodles over the top of the lasagna and spread the remaining 1 cup sauce over the noodles. Sprinkle with the remaining mozzarella, Fontina and 1/4 cup Parmesan.
Cover the dish with foil, tenting it so it doesn’t touch the cheese. Place on a baking sheet. Bake until bubbly around the edges, about 40 minutes. Uncover and bake until browned and crusty on top, 20 to 30 more minutes. Let rest 15 minutes before slicing.
This layering method is different than usual, but it gives the lasagna extra structure: You fold overhanging noodles from the bottom layer over the top!