Heat a large saute pan over medium heat, add olive oil and saute mushrooms for 2-3 minutes, until just beginning to soften. Add mushrooms, garlic, red pepper flakes, oregano and cook for an additional 3-4 minutes, until mushrooms are softened. Add ground beef and cook just until meat is no longer pink, breaking it up with a spatula as it cooks. Add marinara sauce, turn heat to medium-low, and simmer for about 5 minutes. Add salt and pepper to taste.
While the sauce simmers, combine spinach, ricotta and parmesan in a large bowl and mix well.
Take out a 9×13-inch baking dish. Cover the bottom with about 1 1/2 cups marinara sauce. Evenly space 3 no-boil lasagna noodles on top of the sauce layer. Top noodles with about half of the spinach mixture, spreading it into an even layer. Top with about 1/2 of the remaining sauce. Top this sauce layer with 3 more noodles, followed by the ricotta spinach mixture, followed by the remaining noodles. Pour all of the remaining sauce over the noodles and sprinkle with mozzarella cheese. Cover loosely with a piece of aluminum foil.
Bake for 30 minutes (you may want to put your baking dish on a baking sheet to catch any drips), then uncover the lasagna and bake for an additional 10 minutes, or until sauce is slightly bubbly.
Allow to cool for about 10 minutes before slicing.