Preheat & prep. Heat oven to 375°F. Lightly grease a 9×13-inch baking dish.

Cook pasta. Boil ziti in salted water 1–2 minutes less than package directions (al dente). Drain and set aside.

Brown the sausage. In a large skillet over medium-high heat, cook sausage until browned and cooked through (about 7–9 minutes). Break it up as it cooks. Drain excess grease if needed.

Build the sauce. Add onion to the skillet and sauté 3–4 minutes until softened. Add garlic, Italian seasoning, and red pepper flakes; cook 30 seconds. Stir in marinara plus 1/2 cup water/pasta water. Simmer 3–5 minutes.

Mix ricotta layer. In a bowl, combine ricotta, egg, Parmesan, 1 cup mozzarella, and herbs (if using). Season with a pinch of salt and pepper.

Assemble. Toss pasta with most of the meat sauce (reserve about 1 cup for the top). Spread half the pasta mixture into the dish. Dollop and spread the ricotta mixture over it. Add remaining pasta mixture. Spoon reserved sauce on top.

Top & bake. Sprinkle with remaining 1 cup mozzarella and extra Parmesan. Cover with foil (spray foil underside to prevent sticking) and bake 20 minutes. Uncover and bake 10–15 minutes more until bubbly and lightly browned.

Rest & serve. Let stand 10 minutes before slicing. Garnish with fresh basil/parsley if you’d like.