Brown the sausage in a large skillet with olive oil over medium heat for 5-7 minutes, then remove from the pan.

In the same skillet, sauté chopped onion for 4-5 minutes, add minced garlic and optional crushed red pepper, and cook until fragrant.

Mix in tomato paste and stir for 1-2 minutes, then add crushed tomatoes and rigatoni, seasoning lightly with salt and black pepper.

Pour enough chicken stock or water to cover the rigatoni, then simmer uncovered for about 30 minutes, stirring often.

Once rigatoni is tender, return the sausage to the skillet, stir in butter and Boursin cheese until melted.