Heat a large pot or Dutch oven over medium heat. Add the sliced smoked sausage and sauté for about 5 minutes, or until browned.
Add the diced onion, minced garlic, and diced bell pepper to the pot. Cook for another 3-4 minutes, stirring occasionally, until the vegetables have softened.
Stir in the penne pasta, diced tomatoes (with juices), chicken broth, Italian seasoning, red pepper flakes (if using), and season with salt and pepper. Make sure the pasta is submerged in the liquid.
Bring the mixture to a boil, then reduce the heat to low. Cover the pot and let it simmer for about 10-12 minutes, or until the pasta is cooked al dente and has absorbed most of the liquid.
Once cooked, remove the pot from the heat and stir in the grated Parmesan cheese until melted and well combined. Adjust seasoning if necessary.
Serve hot, garnishing with fresh parsley if desired.