1 onion, diced small
1 (8-ounce) package baby bella mushrooms, diced small
1 small (12-ounce) eggplant, diced small
1 small zucchini or yellow squash, diced small
1 red bell pepper, diced small
3 cloves garlic, minced
1 teaspoon salt
1 teaspoon fresh thyme (or 1/2 teaspoon dried thyme)
1 teaspoon fresh oregano, minced (or 1/2 teaspoon dried oregano)
Pinch red pepper flakes, optional
6-ounce package baby spinach
3 cups tomato sauce (or one 28-ounce can crushed tomatoes)
15 dry lasagna noodles (regular noodles, not no-boil)
2 1/2 cups ricotta cheese
3 cups shredded mozzarella cheese
Preheat oven to 400°F.
Warm a tablespoon of olive oil in a large skillet over medium heat. Add the onions and cook until the onions are sweated and are just starting to turn translucent, about 5 minutes. Add the mushrooms and cook until the mushrooms have released their liquid and most of that liquid has evaporated, 5 to 8 minutes more. Add the eggplant, zucchini and bell pepper, and cook until all are softened, another 3 to 5 minutes.
Clear a little space in the middle of the pan. Add another teaspoon of olive oil, the garlic, salt, and spices. Stir the garlic until it is fragrant and just starting to turn golden, about 30 seconds, then stir it into the vegetables. Scatter the spinach over the vegetables and stir until it is wilted, 3 to 5 minutes. Remove the skillet of vegetables from the heat.
Spread a few spoonfuls of the tomato sauce in the bottom of a 9"x13" baking dish. Lay 5 of the lasagna noodles over top, breaking them if needed to create a single layer. Spread 1 cup of the ricotta over the noodles, then half of the vegetables, 1 cup of the tomato sauce, and 1 cup of the mozzarella. Arrange another 5 noodles over the top, followed 1 cup of ricotta, the remaining vegetables, 1 cup of the tomato sauce, and 1 cup of the mozzarella. Top this with a final layer of 5 noodles and the remaining sauce. Dot the top with the remaining 1/2 cup of ricotta. (Reserve the remaining mozzarella for the end of cooking.)
Cover the dish tightly with foil and bake for 1 hour. Check to make sure the noodles are done by poking the lasagna with a knife; the knife should slide easily through all the layers. If it doesn't, cover and cook for another 15 minutes. When the noodles are done, sprinkle the remaining cup of mozzarella over the top of the casserole and bake, uncovered, until the mozzarella has melted, another 10 to 15 minutes.
Turn on the broiler and place an oven rack 6 inches below the heating element. Broil the lasagna until the mozzarella is browned and bubbling, 1 to 2 minutes.
Remove the lasagna from the oven and let it cool on a wire rack for at least 15 minute