Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Bring a large pot of water to a boil. Add 1 tablespoon kosher salt and stir in pasta. Cook until al dente, about 8 minutes.

Heat a skillet over medium-high heat. Add sausage and beef; cook for 5 minutes, stirring to crumble. Add onion, garlic, 3/4 teaspoon salt, 1/2 teaspoon pepper, and red pepper flakes; cook for 3-5 minutes until the onion softens.

Drain pasta, return to the pot, and mix in 1/2 cup marinara sauce. Let sit until needed.

Add remaining marinara sauce and 1/4 cup water to the meat mixture, stir, and simmer for 10 minutes.

In a bowl, mix ricotta, 1/2 cup Parmesan, egg, parsley, basil, remaining salt, and pepper until combined.

Add 2 cups of the meat sauce to the pasta and mix well.

In the prepared baking dish, layer 1/2 cup meat sauce, half of the pasta mixture, 2 cups meat sauce, ricotta mixture, and 1 1/2 cups mozzarella. Add remaining pasta, meat sauce, and mozzarella. Sprinkle with remaining Parmesan.