In the mean time, prepare the filling by chopping spinach and mixing it in a bowl with the 2 cups of mozzarella cheese and ricotta cheese. Set aside.
In a large sauce pan, heat olive oil and saute onion and garlic until translucent. Add beef and cook until beef is no longer pink. Add crushed tomatoes, basil and oregano and season with salt and pepper. Bring to a boil then lower heat and simmer for 10 to 15 minutes.
Cook lasagna noodles according to package instructions.