Preheat oven to 375 degrees F (190 degrees C).
Heat olive oil in a large pot over medium heat. Add the onion and garlic and cook until softened, about 5 minutes.
Add the Italian sausage and ground beef to the pot and cook until browned, breaking up the meat with a spoon as it cooks.
Add the crushed tomatoes, tomato sauce, Italian seasoning, red pepper flakes, salt, and pepper to the pot. Stir to combine and bring to a simmer. Reduce heat to low and simmer for 30 minutes, or until the sauce has thickened.
While the sauce is simmering, cook the pasta according to package directions. Drain the pasta and set aside.
In a large bowl, combine the ricotta cheese, 2 cups of the grated mozzarella cheese, Parmesan cheese, egg, salt, and pepper. Stir to combine.
Add the cooked pasta to the sauce pot and stir to combine.
Spread 1/2 of the pasta mixture in a 9x13 inch baking dish. Top with 1/2 of the ricotta cheese mixture. Sprinkle with 1/2 of the remaining mozzarella cheese.
Repeat layers with the remaining pasta mixture, ricotta cheese mixture, and mozzarella cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Let cool for 10 minutes before serving. Garnish with freshly minced parsley, if desired.