When the water is boiling, add the broccoli and cook until crisp-tender, about 3 minutes. Using the slotted spoon, transfer the broccoli to the bowl with the sausage.
Return the water to a boil, add the pasta, and cook according to the package directions or until al dente. Reserve 1/2 cup of the pasta water, drain, and return the pasta to the pot; set aside.
Return the pan of onions to medium heat, add the cream, and stir, scraping up any browned bits from the bottom of the pan. Bring to a simmer and cook until slightly thickened, about 2 to 3 minutes. Add the sausage and broccoli and stir until combined.