Preheat oven to 375°F (190°C).

Bring a large pot of water to a boil. Add 16 ounce ziti pasta and cook according to package instructions until al dente. Drain the pasta in a colander and set aside.

While the pasta cooks, chop 1 medium onion, mince 2 cloves garlic, and chop 2 medium zucchini.

In the same large oven-safe pot (or a cast-iron pot), heat 2 tablespoons olive oil over medium-high heat. Add the chopped onion and sauté about 5 minutes, until slightly translucent.

Add the minced garlic, chopped zucchini, and 16 ounces spinach to the pot. Cook, stirring, until the spinach is wilted and the zucchini is tender, about 5–7 minutes. Add 1 teaspoon salt and 1/2 teaspoon ground black pepper and stir to combine.

Add 28 ounces crushed tomatoes, 15 ounces tomato sauce, 1 tablespoon Italian Seasoning, and 1 cup ricotta cheese to the pot. Stir and bring the mixture to a gentle simmer.

In a small bowl, combine 1 cup shredded mozzarella and 1 cup shredded Parmesan. Add half of that combined cheese (1/2 cup mozzarella + 1/2 cup Parmesan) into the simmering sauce, then add the cooked ziti and stir well to coat. Remove the pot from the heat.

Sprinkle the remaining cheese (the other 1/2 cup mozzarella and 1/2 cup Parmesan) evenly over the top of the ziti.

Place the oven-safe pot in the preheated oven and bake uncovered for about 10 minutes, until the top cheese is melted and slightly golden brown.

Remove from the oven, let rest for a few minutes, then serve hot.