Heat olive oil in a large pot or deep skillet over medium heat.
Add the sliced Italian sausage and cook until browned, about 5 minutes, stirring occasionally.
Add the minced garlic, chopped onion, and red pepper flakes to the pot. Cook for 2-3 minutes until the onion is translucent and the garlic is fragrant.
Pour in the chicken broth and water, then add the uncooked penne pasta. Stir to combine.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered, stirring occasionally, until the pasta is cooked and most of the liquid is absorbed, about 12-15 minutes.
Stir in the grated Parmesan cheese and chopped fresh parsley. Season with salt and black pepper to taste.
Remove from heat and let sit for 2 minutes before serving.