Steam spinach until tender; press out excess moisture and chop coarsely. Cook mushrooms, onions and minced garlic over medium-high heat until onions are tender; drain excess liquid and cool. Mix ricotta cheese, 2/3 cup Parmesan, egg, salt, pepper, basil and oregano in large bowl. Add cooled spinach and mushroom-onion mixture and mix on low speed until just blended. Cook lasagna according to package directions; rinse under cool water and drain thoroughly. Place four lasagna strips in bottom of lightly oiled 9 by 13 inch pan, overlapping slightly. Top with 2 cups of spinach filling.
Sprinkle with 1 and 1/2 cup shredded cheese and 1/3 cup Parmesan. Repeat layering two more times and top with remaining four lasagna strips. Spread 1 cup of marinara or tomato-cream sauce over top and cover tightly with foil. Preheat oven to 350 degrees and bake, covered, for 1 hour. Remove from oven and keep warm at least 30 minutes before cutting. Top with extra Parmesan cheese. (Can be refrigerated for a day in advance of baking if desired.)