Preheat the oven to 375°F (190 C).
Lightly fry the chopped mushrooms, add garlic powder, salt and pepper.
If you’re using dried lasagna noodles, cook them according to package directions in very salty water until 5 minutes less than al dente. Drain and set aside. I recommend laying the noodles out in a single layer on a lightly oiled baking sheet as soon as they’re cooked - it keeps them from sticking together so they’re easier to work with later!
Pour about 2 tbsp bechamel sauce in the bottom of oven pan. (Or enough sauce to coat the bottom).
Add a layer of lasagna noodles on top in an even layer, tearing them as needed to fit the pan.
Dollop ⅓rd of the spinach/mushroom/cheese mixture over the noodles and spread in an even layer. Followed by 4 tbsp of white sauce. Arrange another layer of noodles on top and repeat process 2 more times.
Arrange one final layer of noodles on top, the last of the white sauce and sprinkle the top with mozzarella and parmesan cheese.
Bake for 30 minutes with foil on and an additional 10 minutes with foil off until tops are browned and bubbly. (You can broil for a few minutes as well, just keep an eye on it so the tops don’t burn).