Prepare all ingredients: measure pasta, trim Brussels sprouts, and mince garlic.
In a large skillet, heat olive oil over medium heat. Add Italian sausage and cook until golden brown, about 5-7 minutes.
Add Brussels sprouts to the skillet and cook for about 5 minutes until tender and slightly caramelized.
Add minced garlic and red pepper flakes (if using) and cook for 1 minute. Pour in chicken broth and simmer for 3 minutes.
Lower heat, stir in heavy cream and Parmesan cheese until melted and creamy. If too thick, add reserved pasta water.
Add cooked penne pasta to the skillet, toss to coat with sauce, and season with salt and pepper. Garnish with fresh parsley before serving.