Cook the lasagna noodles according to package directions. Drain and set aside.
To make the sauce: Heat oil in a large sauce pan over medium heat. Add onions and mushrooms and cook for 10 minutes. Add garlic and spinach. Cover and cook until the spinach is wilted. About 5 minutes. Uncover and cook on medium heat for about 10 minutes or until most of the liquid has evaporated. Add the tomato sauce and pepper.
To make the lasagna: Line the bottom of a 9x13 inch baking dish with 3 noodles. Top with ½ of the ricotta cheese, ½ of the sauce and ½ of the feta cheese. Repeat. Place the final 3 noodles on top and cover with mozzarella cheese.
Bake for 30 minutes or until the cheese is starting to brown.