Follow these steps carefully for baked ziti perfection:

1. Preheat & Prep Pasta Water: Preheat your oven to 375°F (190°C). Bring a large pot of generously salted water to a rolling boil for the pasta. Salting the water is crucial for flavoring the pasta itself.
2. Cook the Pasta: Add the ziti (or your chosen pasta) to the boiling water. Cook according to package directions, but aim for al dente – typically 1-2 minutes less than the package suggests. The pasta will continue to cook in the oven, and you want to avoid it becoming mushy. Drain the pasta well and set it aside. Do not rinse the pasta; the starch helps the sauce cling.
3. Sauté the Aromatics & Vegetables: While the pasta water is coming to a boil or the pasta is cooking, heat the olive oil in a large skillet or Dutch oven over medium heat. Add the chopped onion and sauté until softened and translucent, about 5-7 minutes.
4. Add More Veggies & Garlic: Add the chopped bell peppers and zucchini to the skillet. Continue to cook, stirring occasionally, until the vegetables are tender-crisp, about 7-10 minutes. You want them cooked but still retaining a slight bite. Stir in the minced garlic, dried oregano, dried basil, optional red pepper flakes, salt, and black pepper. Cook for another minute until the garlic is fragrant – be careful not to burn it.
5. Create the Tomato Sauce Base: Pour the crushed tomatoes into the skillet with the vegetables. Stir everything together well. Bring the sauce to a gentle simmer, then reduce the heat to low, cover, and let it simmer for at least 10-15 minutes to allow the flavors to meld. Taste and adjust seasoning (salt, pepper, herbs) if needed.
6. Prepare the Ricotta Mixture: While the sauce simmers, combine the ricotta cheese, the lightly beaten egg, half of the grated Parmesan cheese (1/4 cup), and half of the shredded mozzarella cheese (1 cup) in a medium bowl. Mix well until smooth and thoroughly combined. Season lightly with a pinch of salt and pepper if desired.
7. Combine Pasta, Sauce, and Spinach: If using spinach, stir the chopped spinach into the warm tomato sauce until it begins to wilt (it will continue to wilt in the oven). Remove the sauce from the heat. Add the cooked and drained al dente pasta directly to the skillet or Dutch oven with the sauce (or transfer sauce and pasta to a very large bowl if your skillet isn’t big enough). Stir gently but thoroughly to ensure all the pasta is evenly coated with the veggie-loaded sauce.
8. Assemble the Baked Ziti: Lightly grease a 9×13 inch (or similar large) baking dish. Spread about half of the pasta and sauce mixture evenly across the bottom of the dish.
9. Add the Ricotta Layer: Dollop spoonfuls of the ricotta cheese mixture evenly over the first layer of pasta. You don’t need to spread it perfectly; pockets of creamy ricotta are delicious.
10. Layer Remaining Pasta: Top the ricotta layer with the remaining pasta and sauce mixture, spreading it evenly to cover the ricotta.
11. Cheesy Topping: Sprinkle the remaining shredded mozzarella cheese (1 cup) and the remaining grated Parmesan cheese (1/4 cup) evenly over the top of the pasta.
12. Bake to Perfection: Place the baking dish in the preheated oven. Bake for 20-25 minutes, or until the cheese is melted, bubbly, and lightly golden brown, and the sauce is bubbling around the edges. If you prefer a more browned topping, you can switch the oven to broil for the last 1-2 minutes, but watch it very carefully to prevent burning.
13. Rest Before Serving: Remove the baked ziti from the oven and let it rest for at least 10-15 minutes before cutting and serving. This crucial step allows the cheese and sauce to set slightly, making it easier to serve neat portions and preventing it from being overly soupy.
14. Garnish and Serve: Garnish with fresh chopped basil or parsley, if desired, before serving.