Preheat your oven to 350°F (175°C) and coat a 3-quart casserole dish with non-stick spray.

In a large skillet over medium heat, cook the Italian sausage for 7-10 minutes until browned. Drain excess grease, reserving about 2 tablespoons.

Add diced onions and minced garlic to the skillet with the reserved grease and sauté for 3-5 minutes.

Pour in the diced tomatoes, basil, oregano, salt, and pepper, cooking for an additional 5 minutes.

Incorporate the browned sausage back into the skillet, bring to a gentle boil, then reduce heat and let it simmer for 20-30 minutes.

While the sauce simmers, cook the penne pasta according to package instructions until al dente, about 8-10 minutes.

Combine the cooked pasta with the sausage mixture and half of the mozzarella cheese, stirring over low heat for 2-3 minutes.

Layer half of the pasta mixture in the prepared dish, sprinkle with a layer of mozzarella cheese, then add the remaining mixture and top with more mozzarella.

Cover with aluminum foil and bake for 30 minutes. Remove the foil in the last 5 minutes for a crispy topping.

Allow the casserole to rest for 10 minutes before serving.