adapted from Cook's Illustrated Cover and Bake
Heat 1 tablespoon of oil in a large skillet. Add spinach and cook, stirring, until spinach has wilted. Season with salt and pepper. Transfer cooked spinach to a colander. Squeeze out excess liquid and set aside.
Add another tablespoon of oil and heat up again. Add onions and cook until translucent. Add garlic and cook until fragrant. Transfer to another bowl. Add mushrooms and saute until they are golden. Try to get the mushrooms to release as much moisture as possible. Season with salt and pepper. Transfer to the bowl with the onions and garlic. Add spinach and toss to combine.
Spread 1/2 cup of tomatoes over the bottom of a 9 x 13 inch baking dish (or 2 smaller dishes if you want to freeze half). Lay 3 noodle sheets over the sauce. Make sure the noodles do not touch each other or tough the sides of the dish.
Spread about 2/3 cup of sauce, followed by 3/4 cup of the vegetable mixture and 1 1/3 cup cheese. Repeat this layering of noodles, sauce, vegetables and cheese twice more. For the last layer, lay the remaining three noodles and top with 1 cup tomato sauce and remaining cheese.**
Preheat oven to 375 degrees F. Lightly spray foil with nonstick cooking spray and cover the lasagna. Bake for 25 minutes. Remove the foil and continue to bake until the cheese on top turns golden brown, about 15-20 minutes. Cool for 15 minutes before serving
**After assembling the lasagna in step 4, you can wrap and refrigerate the lasagna for up to one day or freeze for up to one month. If frozen, let the lasagna thaw in the fridge overnight. To bake, let the lasagna sit at room temperature for one hour. Then bake as directed in step 5.