Bring a large pot of salted water to a boil. Cook the rotini pasta according to package instructions until al dente. Reserve about ½ cup of pasta water before draining, then set the pasta aside.

Heat a large, deep skillet over medium-high heat. Add the ground Italian sausage and diced onion. Cook while breaking the sausage apart until browned and no longer pink, about 6–7 minutes. Drain excess grease, leaving about a tablespoon in the pan.

Add the minced garlic, Italian seasoning, and red pepper flakes. Cook for about 1 minute until fragrant.

Pour the undrained Ro*Tel tomatoes and green chilies into the skillet. Scrape up any browned bits from the bottom of the pan. Stir in the chicken broth and heavy cream.

Bring the sauce to a gentle simmer, then reduce heat to medium-low. Cook for about 5 minutes until slightly thickened.

Reduce heat to low. Stir in the Parmesan cheese and shredded Mozzarella or Monterey Jack until melted and smooth.

Add the cooked rotini pasta to the skillet and toss until every piece is coated with the creamy sauce. Add reserved pasta water if needed to loosen the sauce.

Remove from heat. Garnish with fresh chopped parsley and extra Parmesan cheese if desired. Serve immediately.