Bring a large pot of heavily salted water to a rolling boil. Add the penne pasta and cook according to package directions until al dente. Before draining, reserve at least 2 cups of the starchy pasta water, then drain the pasta.
While the pasta cooks, heat a large skillet or Dutch oven over medium-high heat. Add the Italian sausage and cook, breaking it up with a spoon, for 5-7 minutes until browned and cooked through. Use a slotted spoon to transfer the sausage to a plate, leaving the rendered fat in the skillet.
Reduce the heat to medium. Add the chopped onion to the skillet and sauté for 4-5 minutes until softened and translucent.
Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.
Pour in the chicken broth to deglaze the pan, using your spoon to scrape up any flavorful browned bits from the bottom. Let it simmer and reduce for about 2 minutes.
Lower the heat to medium-low and slowly pour in the heavy cream. Stir gently and allow it to warm through without boiling. Stir in the grated Parmesan cheese until it has completely melted and the sauce has started to thicken.
Season the sauce with salt and a generous amount of freshly cracked black pepper.
Return the cooked sausage to the sauce. Add the drained pasta to the skillet and toss to combine.
Pour in about 1/2 cup of the reserved pasta water, stirring continuously until the sauce becomes silky and clings to the pasta. If the sauce is too thick, add another splash of pasta water until you reach the desired consistency.
Let everything simmer together for 1-2 minutes to allow the pasta to absorb the flavors of the sauce. Serve immediately, garnished with fresh parsley and another grind of black pepper.