1. To make the mushroom layer: Heat one tablespoon of butter in a large saucepan and sauté the mushrooms in batches (see note) until browned and no liquid remains in the pan. Season with salt and pepper. Add the garlic and thyme and cook another two minutes, or until the garlic is softened and fragrant. Add the remaining butter and allow it to glaze the mushrooms. Set aside.
2. To assemble the lasagna: Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).
3. Spread 1/4 cup crushed tomatoes in the bottom of a sprayed nine- by 13-inch (20- by 30-centimeter) baking pan and cover with three lasagna noodles. Layer one cup crushed tomatoes and season with 1/2 teaspoon salt and 1/2 teaspoon dried basil. Layer 1/3 cup ricotta cheese (I find it easiest to dot this on) and then four ounces shredded mozzarella cheese. Add another lasagna layer. Cover with the sautéed mushrooms. Dot on another 2/3 cup ricotta and cover with another four ounces shredded mozzarella cheese. Add another lasagna layer and an additional one cup crushed tomatoes. Season with 1/2 teaspoon salt and 1/2 teaspoon dried basil. Cover with the rest of the ricotta and the last of the mozzarella. Sprinkle the Parmesan over the top, if using.
4. Spray a piece of foil and cover the pan tightly. Bake for 25 minutes covered and 15 minutes uncovered or until browning and bubbly.
5. Alternatively, refrigerate raw for up to 24 hours, then bake 60 minutes covered (to compensate for the cold) and 15 minutes uncovered.
To get maximum flavor from the mushrooms, allow them to brown in batches. I do it in thirds and push the browned mushrooms to one side of the pan while the new batch cooks. Remember, if there’s water in the pan, the mushrooms aren’t done!