Preheat the oven to 375°F (190°C).
Cook the ziti according to package directions until al dente. Drain and set aside.
Heat olive oil in a large skillet over medium heat.
Add the ground beef and onion. Cook until the meat is browned and the onion is softened.
Stir in the garlic and cook for 1 minute.
Add the marinara sauce, diced tomatoes, Italian seasoning, salt, and pepper. Simmer for 10 minutes.
In a separate bowl, combine the ricotta cheese, egg, Parmesan cheese, and 1 cup of mozzarella cheese.
In a large mixing bowl, combine the cooked ziti with the meat sauce.
Spread half of the pasta mixture into a greased 9x13-inch baking dish.
Spoon the ricotta mixture over the pasta and spread gently.
Top with the remaining pasta mixture.
Sprinkle the remaining mozzarella cheese evenly over the top.
Bake uncovered for 25 to 30 minutes, or until the cheese is melted and bubbly.
Allow the baked ziti to rest for 10 minutes before serving.