Preheat the oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions; drain and set aside.
Heat olive oil in a large pan over medium heat; add chopped onion and garlic, cooking until softened.
Add ground beef or sausage to the pan; cook until browned, breaking up the meat as it cooks. Drain excess fat.
Stir in the crushed tomatoes, tomato paste, oregano, basil, salt, and pepper; simmer for about 20 minutes.
In a bowl, mix ricotta cheese, egg, salt, and pepper until combined.
Start layering by spreading a thin layer of tomato sauce on the bottom of a baking dish, followed by noodles, then ricotta, mozzarella, and Parmesan.
Repeat the layering process until all ingredients are used, finishing with mozzarella and Parmesan on top.
Cover the baking dish with aluminum foil and bake for 25 minutes; remove the foil and bake for an additional 15 minutes.
Let the lasagna rest for 10-15 minutes before slicing; garnish with fresh parsley.