Cook the ziti according to the package instructions. Heat oven to 400°F.

In a large pot, over medium-low heat, heat the oil. Add tbe onion, salt, and pepper. Cover and cook until the onion is softened, 5-7 minutes. Add beef, increase heat to medium-high, and cook until no trace of pink remains, 5-8 minutes. Drain any remaining liquid. Add the garlic and oregano (if using) and cook for 2 minutes. Add the pasta sauce and heat for 3 minutes. Remove from heat. Add cooked pasta and toss to coat. Add parmesan, ricotta, and spinach and toss again. Spread mixture into a 9x13 inch baking dish or individual ramekins and sprinkle with the mozzarella. Bake until the mozzarella melts, about 15 minutes.