Preheat the oven to 350°F (177°C).

For the sauce, heat the olive oil in a large, high-sided skillet over medium-high heat. Add the chopped onion and sauté until translucent, about 5–7 minutes, reducing the heat as necessary to avoid browning. Lower the heat to medium-low, add minced garlic, and cook for an additional 1–2 minutes until fragrant. Incorporate the ground beef, salt, and pepper, cooking until the meat is no longer pink; drain excess fat if needed. Stir in the spaghetti sauce and Italian seasoning, combining thoroughly.

For the ricotta filling, in a large mixing bowl, blend together the ricotta cheese, eggs, nutmeg, Italian seasoning, shredded mozzarella, and grated Parmesan until smooth and fully incorporated.

Prepare the pasta by cooking the ziti according to the package instructions until al dente. Drain well.

To assemble, spread 1/2 cup of the meat sauce across the bottom of a 13x9-inch baking dish. Layer half of the cooked pasta evenly over the sauce, followed by half of the remaining meat sauce, then dollop and spread half of the ricotta mixture over the top. Repeat these layers, finishing with the ricotta layer.

Place the dish onto a rimmed baking sheet to catch any potential overflow. Cover the dish with foil and bake for 40 minutes, or until the internal temperature reaches 160°F. Remove the foil, sprinkle the remaining 2 cups of mozzarella cheese over the top, and bake uncovered for an additional 10 minutes until the cheese is melted and bubbly. Let the baked ziti rest for 15 minutes before slicing and serving.