Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish and set it aside.
Cook the ziti pasta according to the package instructions until it's just shy of al dente (it will continue to cook in the oven). Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, and sauté for about 2-3 minutes until they become fragrant and slightly translucent.
Add the diced bell pepper, zucchini, and mushrooms (if using) to the skillet. Sauté the vegetables for about 5-7 minutes until they start to soften.
Pour in the crushed tomatoes and diced tomatoes (with their juices) into the skillet. Add the dried basil, dried oregano, salt, and black pepper. Stir well, and let the sauce simmer for about 10-15 minutes, allowing the flavors to meld and the sauce to thicken slightly.
In a large mixing bowl, combine the cooked ziti pasta with half of the tomato sauce. Mix until the pasta is well-coated.
Spread half of the pasta mixture into the prepared baking dish. Top with dollops of ricotta cheese and half of the shredded mozzarella cheese.
Layer the remaining pasta mixture on top, followed by the remaining mozzarella cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.
Allow the meatless ziti to cool slightly before serving. Garnish with fresh basil leaves if desired.