Chop onions and garlic. Heat oil in skillet; add onions, garlic, and ground beef. Mix together, breaking up beef, and cook until beef is lightly browned. Pour off excess fat. Add tomatoes, tomato sauce, oregano, salt, and pepper. Simmer for about 45 minutes, stirring occasionally, until some of the liquid has evaporated and mixture is thickened to sauce consistency.
Meanwhile, cook lasagna in boiling salted water, following package directions. Cook until tender; drain thoroughly and rinse with cold water until cool enough to handle.
In a large baking pan (13x9x2-inch) spread 1/3 of the sauce in the bottom. Cover with a layer of lasagna, using about 1/2 of the noodles. Spread with half of the ricotta cheese, then top that with about 1/3 of the mozzarella slices; sprinkle with Parmesan cheese. Repeat with the next layer then top with remaining 1/3 of sauce.
Bake at 350° for 25 to 30 minutes, or until hot and bubbly. Top with remaining cheese about halfway through baking. Cut in squares to serve.
Serves 8 to 10
Calories per serving, based on 8 servings: 711