Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente. Drain and set aside.

In a skillet over medium heat, add the Italian sausage. Cook until browned, breaking it apart with a spoon.

Add the chopped onion, garlic, and bell pepper to the skillet. Sauté until the vegetables are softened, about 5 minutes.

Stir in the diced tomatoes, oregano, basil, red pepper flakes, salt, and pepper. Simmer for 10 minutes.

Add the cooked pasta to the skillet and toss to combine with the sauce. Cook for an additional 2 minutes to heat through.

Serve hot, garnished with grated Parmesan cheese and chopped parsley.