Heat olive oil in a large deep skillet or pot over medium heat.
Add the chopped onion and sauté for 3-4 minutes until translucent.
Add the minced garlic and cook for 1 minute until fragrant.
Add the dried penne pasta to the pot and stir to coat with the oil, onion, and garlic.
Pour in the chicken broth, whole milk, and heavy cream.
Add salt, black pepper, and crushed red pepper flakes, then stir to combine.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered.
Cook for 15-18 minutes, stirring occasionally, until the pasta is tender and most of the liquid is absorbed.
Remove the pot from heat and stir in the grated Parmesan cheese and unsalted butter until melted and creamy.
Sprinkle chopped fresh parsley on top before serving.