In a large skillet, heat olive oil over medium heat. Sauté onion until soft and translucent, about 5 minutes, then add garlic and cook until fragrant for 1 minute. Add ground beef, browning until fully cooked for about 7-10 minutes. Stir in crushed tomatoes, tomato sauce, tomato paste, sugar, oregano, basil, salt, pepper, and optional red pepper flakes. Simmer on low for 20-30 minutes, until thickened.

While the beef sauce simmers, combine ricotta cheese, egg, grated Parmesan, Italian seasoning, and a pinch of salt and pepper in a mixing bowl. Set the mixture aside in the refrigerator.

Bring a large pot of salted water to a rolling boil. Cook lasagna noodles until al dente, about 8-10 minutes. Drain and lay flat on a kitchen towel to cool.

Preheat oven to 375°F (190°C). Spread a thin layer of beef sauce on the bottom of a baking dish. Layer noodles, followed by ricotta filling and beef sauce. Repeat until all ingredients are used, finishing with noodles topped with beef sauce and mozzarella.

Cover with aluminum foil and bake for 25 minutes. Remove foil and continue baking for another 15-20 minutes until cheese is golden and edges are bubbling.

Allow lasagna to rest for 10-15 minutes before slicing. Garnish with freshly chopped parsley.