Prepare spaghetti according to package directions. Preheat oven to 350 degrees F.
In a medium skillet over medium heat, brown hamburger. Add onion, salt and pepper and cook until the onion is soft. Drain off excess juices.
In a medium bowl, beat the eggs. Add the milk and one cup of mozzarella cheese and mix. Toss mixture with cooked pasta.
Grease a 9x13x2 inch pan with non-stick cooking spray of butter. Spread pasta mixture in the baking dish.
Layer ground beef, pasta sauce, remaining 3 cups of mozzarella cheese, black olives and mushrooms. Cover and bake at 350 degrees F for 20 minutes. Remove cover and bake for another 10 minutes. Remove from oven and let stand for 5 minutes before serving.