Bring a large pot of well-salted water to a boil. Cook the rigatoni until just al dente according to the package directions, reserve about ½ cup of pasta water, and drain.

While the pasta cooks, heat the olive oil in a large skillet or Dutch oven over medium heat. Add the sliced smoked sausage and cook for 4–5 minutes, turning occasionally, until lightly browned.

Transfer the sausage to a plate if the pan becomes crowded. Add the diced onion and bell pepper to the same pan and cook for 4–5 minutes, until softened.

Stir in the minced garlic and cook for about 30 seconds, just until fragrant. Add the crushed tomatoes, Italian seasoning, salt, black pepper, and optional red pepper flakes.

Return the browned sausage to the pan. Simmer the sauce uncovered for 10–12 minutes, stirring occasionally, so it thickens and absorbs the sausage flavor.

Add the drained rigatoni to the sauce and toss until every piece is coated. Add a small amount of reserved pasta water if the sauce is too thick.

Stir in the finely grated Parmesan while the pasta is hot. Garnish with chopped parsley or basil and serve immediately.