Preheat oven. Preheat the oven to 350°F. Spray a 9×13-inch baking dish with nonstick cooking spray and set aside.: The oven should be hot and even so the mozzarella cheese bubbles and browns, giving a golden top and a hot interior. You will notice the oven steadying at a warm, drying heat of 350°F, which crisps edges without burning. If the oven temperature is uneven, rotate the dish halfway through the bake to avoid hot spots. A common mistake is placing the dish in a too cool oven, which prevents proper browning and leads to long, soggy baking times.
Prepare pasta. Boil 1 pound rigatoni according to package instructions. Drain.: As you boil the rigatoni , the water should chatter with rolling bubbles, and you should time the pot to deliver pasta that is slightly firm to the bite. Drain when the center offers a faint resistance, because the pasta will continue to cook in the oven. Overcooking at this stage yields mushy tubes that collapse, while undercooking leaves a chalky center.
Make sauce. In a large pot, heat 1 tablespoon olive oil over medium heat. Stir in 1 cup onion and 4 cloves garlic and sauté 3 to 5 minutes, or until onions are tender. Add in 1 pound Italian sausage or beef and cook 5 to 7 minutes, or until browned. Drain excess grease, if needed. Stir in 15 ounces crushed tomatoes, 26 ounces marinara sauce, 2-4 tablespoons fresh basil, 2 teaspoons Italian seasoning, 1 teaspoon salt, and 1/4 teaspoon pepper. Simmer 10 to 15 minutes.: When the olive oil shimmers in the pot, add the onion and sauté until it softens and smells sweet, then stir in the garlic until fragrant. Browning the Italian sausage creates dark flecks and a toasty scent, which infuses the sauce with depth. After adding the crushed tomatoes and marinara sauce , the mixture should go from bright red to a slightly darker, cohesive sauce as it simmers, smelling richer and rounder. Simmering for 10 to 15 minutes reduces acidity and concentrates flavor, yielding a sauce that clings to pasta. Watch for splattering, and skim excess grease after browning the meat to avoid a greasy finish. A frequent issue here is rushing the simmer, which leaves the tomatoes sharp and underdeveloped.
Assemble and bake. Stir in cooked rigatoni. Spoon 1/2 of the mixture into the baking dish. Top with 1/2 of the mozzarella cheese. Spoon the remaining ziti mixture over the cheese. Top with the remaining cheese (add more if desired). Bake 20 to 25 minutes, or until cheese is bubbly.: Stir the cooked rigatoni into the sauce so each tube is glossy and coated, then spoon half the mixture into the prepared dish. The first layer should look saucy yet structured, not soupy, and a thin glisten of oil may appear. Sprinkle half the mozzarella cheese in an even layer so it melts into the pockets of pasta, then top with the remaining saucy pasta and finish with the rest of the mozzarella cheese . As it bakes, listen for gentle bubbling and watch for the cheese to transform from soft strands to browned, blistered patches. If the top is browning too quickly, tent loosely with foil to prevent burning while the center finishes heating through. A common pitfall is adding too much sauce so the dish becomes watery; aim for saucy but not soupy.
Serve. Top with fresh basil or parsley, if desired. Serve hot.: After removing from the oven, allow the dish to rest for a few minutes so the sauce settles and slices hold together when served. The aroma should be warm and herbaceous, with the basil brightening each forkful. When plating, expect strings of mozzarella cheese and a satisfying contrast between the tender rigatoni and the slightly crisped edges. Serving immediately while still warm preserves the ideal texture; letting it sit too long will cause the cheese to firm up and become less glossy.