If you have oodles of time on your hands, or live near a Wegman's, you could use roasted or even grilled marinated veges instead of just mushrooms. Carrot, onion, zuccini, and even tomato could be roasted or grilled and make an excellent filling for this dish. Mess around and find your favorite combination. I know I am a slow cook, but this dish took me a whopping 2 hours to put together and bake so I won't be messing around with it much.
1lb lasagna noodles
15 oz whole milk ricotta cheese
4 cups milk
2 lbs Portabella mushroom caps
10 oz frozen chopped spinach
2 cloves garlic, smashed
½ cup grated parmesan plus extra for sprinkling top
1 stick unsalted butter plus another ½ stick to cook mushrooms
½ cup all purpose flour
¾ tsp fresh ground black pepper
1 tsp nutmeg
1 tbs kosher salt
Set oven to 375F
Boil lasagna in salted water for 10 minutes, until pliant but firm to the bite. Drain and mix with oil to prevent sticking and to keep moist.
Soak the spinach in bowl of hot water and break up with your fingers, grab a palm full and squeeze the hell out of it with a bundle of paper towels. Squeeze until no more water comes out, set aside and repeat with the rest using fresh towels until the spinach is VERY dry.
Sauté the mushrooms in 2 batches with a couple tablespoons butter, salt until tender, about 5 minutes. Use 1 garlic clove per batch. Set aside off the heat.
Melt 1 stick unsalted butter in a saucepan over medium low heat, add flour and whisk quickly to remove any lumps that may form. Cook this roux, stirring constantly for about 2 minutes or until bubbles form at the edges. Add the milk, salt, pepper and nutmeg all at once and continue to stir until the sauce thickens enough to coat the back of a spoon. Keep the heat at or below medium and stir constantly to prevent burning. Set aside off the heat.
Mix the ricotta in a bowl with the egg, ½ cup parmesan, dried spinach and dash salt and pepper. Mix in about 1/3 cup of the milk sauce to loosen.
Pour enough sauce in the bottom of a 9x13x2 or 3 inch baking dish to lightly coat the bottom. Put down a layer of noodles (about 5), then about ¾ cup of the sauce (enough to coat noodles), then 1/3 of the mushrooms, then 1/3 of the ricotta mixture spread evenly. Repeat with 2 more of these layers and finish with layer of noodles, the rest of the sauce and sprinkle with parmesan. Bake for 45 minutes, cover with foil if top gets too brown.