Heat the oven to 350°F and lightly grease a 9-by-13-inch baking dish.

Bring a large pot of salted water to a boil. Cook the ziti for 1–2 minutes less than the package directions specify for al dente, then drain it well.

Heat the olive oil in a large skillet. Cook the sausage or ground beef with the onion for about 6–8 minutes, until browned and fully cooked, then drain excess fat if needed.

Add the garlic and cook for about 30 seconds. Stir in the marinara sauce and Italian seasoning, then simmer for 5 minutes.

Mix the ricotta, egg, Parmesan, sour cream, salt, pepper, and half of the mozzarella in a bowl.

Combine the drained pasta with about two-thirds of the meat sauce. Fold in the ricotta mixture gently so the pasta stays intact.

Spread a little sauce in the baking dish, add half of the pasta mixture, and layer on some sauce and mozzarella. Add the remaining pasta, top with the remaining sauce, and finish with the remaining mozzarella.

Cover the dish loosely with foil and bake for 20 minutes. Remove the foil and bake for another 10–15 minutes, until the casserole is bubbling and lightly browned.

Let the baked ziti rest for 10 minutes before serving. Garnish with chopped parsley or basil if desired.