1/2 cup chopped fresh oregano or 1 1/2 tablespoons dried oregano (optional)
1 24- to 26-ounce jar pasta sauce
1 15-ounce container ricotta
1 10-ounce box frozen spinach thawed and squeezed to remove excess moisture
1/2 cup (2 ounces) grated Parmesan
1 cup (4 ounces) shredded mozzarella
Heat oven to 400° F. Cook the ziti according to the package instructions. Meanwhile, heat the oil in a large pot over medium heat. Add the onion, salt, and pepper and cook until softened, 5 to 7 minutes. Add the beef and cook, crumbling it with a spoon, until no trace of pink remains, about 7 minutes. Spoon off and discard any excess fat. Add the garlic and oregano (if using) and cook for 2 minutes. Add the pasta sauce and cook for 3 minutes. Remove from heat. Add the drained ziti to the pot and toss. Add the ricotta, spinach, and 1/4 cup of the Parmesan and toss again. Spread the mixture into a 9-by-13-inch baking dish. Sprinkle with the mozzarella and the remaining Parmesan. Bake until the mozzarella melts, about 15 minutes.
To freeze: Assemble — but do not bake — the casserole. Cover tightly with 2 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.
Tip: If you prefer, substitute Italian sausage for the ground beef and chopped broccoli for the spinach.