Bring a large pot of salted water to a boil and cook the rigatoni according to package instructions until al dente. I always reserve a small cup of pasta water before draining, just in case the sauce needs thinning later. Drain the pasta and set it aside.
Heat olive oil in a large skillet over medium heat. Add the sausage and cook while breaking it into small crumbles. Let it brown well, as this adds deep flavor to the entire dish.
Once the sausage is cooked through, stir in the chopped onion. Cook until it becomes soft and translucent, then add the garlic. Let it cook briefly until fragrant, being careful not to burn it.
Add the sliced mushrooms to the skillet and stir well. Cook until they release their moisture and turn golden brown. This step is important for developing their rich, savory taste.
Pour in the chicken broth and let it simmer for a minute to deglaze the pan. Slowly add the heavy cream, stirring gently. Allow the sauce to simmer until it slightly thickens and coats the back of a spoon.
Stir in the freshly grated Parmesan cheese until melted and smooth. Season with salt and black pepper to taste. If the sauce feels too thick, add a splash of reserved pasta water.
Add the cooked rigatoni to the skillet and toss gently until every piece is coated with sauce. Let everything cook together for a minute so the flavors can fully blend.
Turn off the heat and sprinkle chopped parsley over the pasta. Give it a final gentle toss before serving warm.