First, get your oven ready by preheating it to 350°F. While it’s heating up, grab your 3-quart casserole dish and coat it with a non-stick cooking spray. This simple step ensures that your casserole won't stick, making serving and cleanup much easier later.

Start by removing the Italian sausage from its casings and adding it to a large skillet. Cook it over medium heat, using a spoon to crumble the meat as it browns. Once cooked through, drain the grease, but be sure to reserve about two tablespoons in the skillet.

Spoon the reserved sausage grease back into the skillet. Add the chopped onion and garlic, and sauté until the onion becomes soft and translucent. Now, pour in the can of diced tomatoes and stir in the basil, oregano, salt, and pepper to create a flavorful base.

Add the cooked sausage back into the skillet with the tomato and onion mixture. Bring everything to a gentle boil, then reduce the heat to low. Cover the skillet and let the sauce simmer for 20 to 30 minutes, stirring occasionally, to allow the flavors to meld together.

While your sauce is simmering, cook the penne pasta in a large pot of salted water according to the package directions for al dente. Cooking it al dente is key, as the pasta will continue to cook in the oven. Once done, drain the pasta well and return it to the pot.

Pour the sausage and tomato sauce mixture over the drained pasta in the pot. Add half of the grated mozzarella cheese and stir everything together until the cheese is melted and well combined. This ensures every bite has that cheesy goodness mixed right in.

Transfer half of the pasta and meat mixture into your prepared casserole dish. Sprinkle a layer of cheese over it, then add the remaining pasta mixture on top. Finish by covering the entire casserole with the rest of the mozzarella cheese, cover with foil, and bake for about 30 minutes, or until bubbly.