In a skillet over medium-high heat, cook, onions, and garlic in olive oil until onions are tender. Drain excess liquid and cool. Boil spinach for 5 minutes. Drain, then squeeze out excess liquid. Chop spinach.
Combine ricotta cheese, Parmesan cheese, spinach, salt, oregano, basil, pepper, and egg in a bowl. Lay 5 lasagna sheets in bottom of a 9x13 inch baking dish. Spread one third of the cheese/spinach mixture over noodles. Sprinkle 1 cup mozzarella cheese. Spread 1 cup tomato sauce over cheese. Repeat layering 2 times.
Grate cheese on top of final layer and place in oven and cook for 50 minutes.