Preheat the oven to 350°F.
Bring a large pot of salted water to a boil. Add the penne pasta and cook it just until al dente, about 10 minutes. Drain the pasta and set it aside.
Add the olive oil to a large heavy-bottomed stock pot set over medium-low heat. Once the oil is warm, add the minced garlic, diced onions, dried oregano, and a pinch of salt and pepper and cook, stirring constantly, until the onions are translucent. Add the Italian sausage and cook, breaking apart the sausage with a spatula, until the sausage is browned and cooked throughout.
Drain any grease from the pot, reduce the heat to low then add the tomato sauce and tomato paste. Simmer the sauce, stirring occasionally, for 10 minutes. Add the cooked pasta to the pot and stir to combine.
Transfer half of the the pasta to a 9x13-inch baking dish, top it with half of the sliced mozzarella cheese, then repeat another layer of the pasta and the sliced cheese.
Bake the pasta for about 15 minutes or until the cheese is melted. Remove the baked penne from the oven, garnish it with the basil leaves and shredded Parmesan cheese and serve.