Heat olive oil in a large pot or deep skillet over medium heat.
Add the crumbled Italian sausage and cook until browned and cooked through, about 5-7 minutes, breaking it up with a spoon as it cooks.
Add the diced onion to the pot and sauté until softened, about 3 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juices, chicken broth, and water. Stir to combine.
Add the uncooked penne pasta, dried Italian seasoning, crushed red pepper flakes, salt, and black pepper.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered, stirring occasionally, until the pasta is tender and most of the liquid is absorbed, about 15 minutes.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and well combined.
Garnish with chopped fresh parsley before serving.