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

Cook the ziti in a large pot of heavily salted boiling water according to package directions until al dente (usually 1-2 minutes less than the package time). Before draining, reserve about 1 cup of the starchy pasta water. Drain the pasta and set aside.

While the pasta cooks, make the sauce. Heat the olive oil in a large skillet or Dutch oven over medium-high heat. Add the chopped onion and cook until softened, about 5-6 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the ground beef to the skillet. Break it up with a spoon and cook until browned, about 5-7 minutes. Drain off any excess fat.

Stir in the tomato paste and cook for 1 minute. Add the crushed tomatoes, oregano, basil, red pepper flakes (if using), sugar, salt, and pepper. Bring the sauce to a simmer, then reduce the heat to low, cover, and let it simmer for at least 15-20 minutes to allow the flavors to meld.

In a medium bowl, combine the ricotta cheese, Parmesan cheese, beaten egg, and chopped fresh parsley. Season with a pinch of salt and pepper and mix until well combined.

In a large bowl, combine the cooked ziti with about 2 cups of the meat sauce. If the mixture seems too dry, add a splash of the reserved pasta water. Mix gently to coat the pasta.

Spread about 1 cup of the remaining meat sauce evenly over the bottom of the prepared baking dish.

Spoon half of the pasta and sauce mixture into the baking dish. Drop spoonfuls of the ricotta mixture over the pasta layer. Sprinkle with half of the shredded mozzarella cheese (about 2 cups).

Top with the remaining pasta mixture. Spread the last of the meat sauce over the top, ensuring it covers the pasta. Sprinkle evenly with the remaining 2 cups of mozzarella cheese.

Cover the baking dish with aluminum foil. Bake for 20 minutes. Remove the foil and continue to bake for another 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden on top.

Remove from the oven and let the baked ziti rest for at least 10 minutes before serving. This allows it to set and makes for cleaner slices. Garnish with fresh parsley before serving.