Preheat your oven to 375°F.
Bring a large pot of salted water to a boil and cook the penne until just al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add diced onion and cook until soft and fragrant. Stir in garlic and let it cook just until golden.
Add the Italian sausage and break it apart as it browns. Cook until fully done and slightly crisp around the edges.
Pour in the marinara sauce and stir everything together, letting it simmer for a few minutes so the flavors can blend.
Add the cooked pasta into the sauce and gently toss until every piece is coated.
Transfer to a baking dish and top generously with mozzarella and parmesan cheese.
Bake uncovered for about 20 minutes, or until the cheese is melted, bubbling, and lightly golden on top.
Finish with fresh basil right before serving.