Pasta: Bring a large pot of water to a boil, then add a generous spoon of salt (it should taste like ocean water). Add pasta and cook according to the instructions on the package, until al dente. When finished, drain and set aside.
Veggies: Prep all veggies, finely dicing them so they're about the same size. Add oil to a large saute pan over medium heat. Add zucchini, eggplant, pepper, onion, and garlic, cooking until they're slightly softened, 5 to 7 minutes.
Sauce: Add marinara and oregano to the veggies, cover, and let gently simmer for 15 minutes. Stir in basil, and add salt to suit your taste, about ¼ tsp. Stir 3 cups of the vegetable sauce into the pot of pasta, leaving remaining sauce in the pan.
Ricotta Mix: Stir together ricotta, thawed spinach, and parmesan. Fold ricotta mixture into pasta, leaving large clumps of ricotta.
Cheese: Stir together mozzarella and parmesan.
Assemble: Preheat oven to 350°F (175°C). Spread ½ of the sauce onto the bottom of a 9×13 inch (or similar sized) casserole dish. Then layer ½ of the pasta > ½ of the cheese > remaining ½ of the pasta > remaining ½ of the sauce > remaining ½ of the cheese.
Baked: For about 30 minutes, or until cheese is golden and melted, and sauce is bubbling around the outside. If the cheese browns too quickly, cover the dish with aluminum foil. Allow to rest for a few minutes before serving.
Store in an airtight container in the refrigerator for about 5 days.
Make ahead by either making the components and storing them separately, or assembling and sealing tightly. Can be prepped 24 hours in advance.
Can be frozen to be baked later. Freeze prepared ziti for up to a month. When ready to cook, allow the ziti to thaw in the refrigerator, then bake as directed.