Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with olive oil or non-stick cooking spray.
Cook the penne pasta according to package directions for al dente, then drain thoroughly and return it to the empty pot.
While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it up with a wooden spoon, for 5-7 minutes until it is browned and no longer pink. Add the chopped onion and cook for another 4-5 minutes until softened, then stir in the minced garlic and cook for 30 seconds until fragrant.
Pour the jar of marinara sauce and 1 cup of water into the skillet with the sausage mixture. Stir to combine, bring to a simmer, then reduce the heat and let it cook for 5 minutes to allow the flavors to meld. Season with 1/2 teaspoon of salt and 1/4 teaspoon of pepper.
Pour the sausage and sauce mixture over the drained pasta in the pot and stir until the pasta is evenly coated. Transfer half of this mixture to the prepared baking dish and sprinkle with half of the mozzarella cheese. Add the remaining pasta mixture and top with the rest of the mozzarella and the 1/4 cup of grated Parmesan.
Cover the dish tightly with aluminum foil and bake for 20 minutes. Remove the foil and continue baking for another 10-15 minutes, or until the cheese is completely melted and the edges are bubbly.
Remove the bake from the oven and let it stand for 5-10 minutes before serving. This allows the sauce to set slightly for easier serving. Garnish with the chopped fresh basil.
