Steam spinach until tender then press out excess moisture and chop coarsely. Mix mushrooms, onions and garlic and saute 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 then rinse under cool water and drain thoroughly. Place four lasagna strips in bottom of lightly oiled 9x13" pan, overlapping slightly. Top with 2 cups spinach filling. Sprinkle with 1 1/2 cups shredded cheese and 1/3 cup parmesan. Repeat layering two more times and top with remaining four lasagna strips. Spread 1 cup of marinara sauce over top (make sure all noodles have sauce on them so they don't dry out) and cover tightly with foil.
Preheat oven to 350F and bake, covered, for 1 hour. Turn oven off and allow lasagna to sit for at least 30 minutes before removing from oven. Allow 10 minutes wait time before cutting. Top with extra Parmesan cheese. (Can be refrigerated for up to a day in advance of baking, if desired.).