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 the onion, salt, and pepper. Cover and cook until the onion is transparent, 10-12 minutes. 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 the cooked pasta and toss to coat. Add the Parmesan, ricotta, and spinach and toss again. Spread the mixture into a 9-by-13-inch baking dish or individual ramekins and sprinkle with the mozzarella. Bake until the mozzarella melts, about 15 minutes.
Tip: If you prefer, substitute chopped broccoli for the spinach.
To Freeze: Assemble (but do not bake) the casserole. Cover tightly with two layers of aluminum foil. Store for up to 3 months.
To Reheat: Thaw overnight in the refrigerator or thaw partially in the microwave. Cover and heat in a 350° F oven for 1 hour. Uncover and heat until the mozzarella melts, about 10 minutes more.