Cook lasagna noodles according to package directions.

Preheat oven to 350°F (175°C).

Heat olive oil in a large skillet over medium heat.

Add chopped onions and minced garlic to the skillet and sauté until tender.

Stir in tomato sauce, salt, and red pepper.

Bring the sauce to a boil, then reduce heat, cover, and simmer for 1 hour, stirring occasionally.

In a separate skillet, sauté sliced sausages (or ground beef) until browned.

Drain off any excess fat.

In a bowl, mix together ricotta cheese and diced mozzarella cheese.

Spread 1/2 cup of the tomato sauce on the bottom of a baking pan.

Arrange a layer of cooked lasagna noodles over the sauce.

Top the noodles with a layer of the ricotta cheese mixture.

Sprinkle with parmesan cheese.

Spoon tomato sauce over the cheese and spread evenly.

Add a layer of sautéed sausage (or ground beef).

Repeat the layering process: noodles, cheese mixture, parmesan cheese, tomato sauce, and meat.

Finish with a final layer of noodles and tomato sauce.

Bake in the preheated oven for 20 minutes, or until the lasagna is hot and bubbly.