Heat oil in a large saucepan over medium-high heat. Add onion, 1 teaspoon salt, and garlic; cook until softened, about 3 minutes. Add ground beef and sausage and cook, crumbling with a spoon, until browned, about 5 minutes. Stir in 3 cups marinara sauce; bring to a simmer. Reduce heat to low and simmer while pasta cooks.

Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a large casserole dish.

Bring a large pot of salted water to a boil. Add pasta and cook until just tender with a bite, about 7 minutes. Drain; return spaghetti to the pot. Stir in 3 1/2 tablespoons butter and remaining marinara sauce.

Combine cottage cheese, cream cheese, sour cream, remaining salt, pepper, and 1 cup mozzarella in a medium bowl and stir until well incorporated.

Add half of the pasta to the casserole dish. Spread cheese mixture evenly over pasta. Add remaining pasta; cut remaining 1 1/2 tablespoons butter into thin slices, and arrange over pasta. Top with meat sauce mixture, remaining 2 cups mozzarella, and grated Parmesan cheese. Place casserole dish on a rimmed baking sheet.