In a large sauce pan, brown and crumble beef over medium heat until no longer pink. Drain any grease.
Add olive oil and chopped onions. Cook until onions are translucent, about 3-4 minutes.
Stir in minced garlic and cook another minute until garlic is fragrant.
Add crushed tomatoes, Italian seasoning, dried basil and oregano, sugar, salt and red pepper flakes. Simmer for 5 minutes and remove from heat.
In a medium mixing bowl, combine ricotta cheese, egg and grated parmesan cheese.
In a deep 9x13 casserole dish, add 2 cups of sauce and spread it out in a even layer.
Top with 4 lasagna noodles.
Spread ⅓ of the ricotta cheese mix over the noodles.
Sprinkle 1 cup of shredded mozzarella cheese on top.
Repeat layers: 2 cups sauce, 4 lasagna noodles, ⅓ ricotta cheese, 1 cup shredded mozzarella.
Repeat layers again: 2 cups sauce, 4 lasagna noodles, remaining ricotta cheese and 1 cup shredded mozzarella.
Top with 2 cups of sauce and 4 more noodles.
Then add the remaining sauce on top of the noodles and the remaining mozzarella cheese on top of the sauce.
Cover with aluminum foil and let rest while the oven preheats to 375°F.
Bake covered for 30 minutes, uncover and bake for 15-20 minutes until the cheese is started to turn golden and the sauce is bubbling.
Let rest 20 minutes, then slice and serve.