Preheat the oven to 375°F and lightly grease a baking dish. Cook the penne pasta in a large pot of salted water until al dente, then drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add the Italian sausage and cook until browned, breaking it up with a spoon as it cooks.
Add the sliced bell peppers, onion, and minced garlic to the skillet. Cook for 5 minutes until the vegetables soften.
Stir in the marinara sauce, Italian seasoning, salt, and pepper. Simmer for 3 minutes to combine the flavors.
Remove the skillet from heat and mix in the cooked pasta until evenly coated. Transfer everything to the prepared baking dish.
Sprinkle the mozzarella cheese and Parmesan cheese evenly over the top. Bake for 20 minutes until the cheese is melted and lightly golden.