Heat oil over medium-heat in a oven-safe skillet until smoking.
Add sausage and onion, and cook until lightly browned, about 4 minutes. Add in the garlic, and cook for 30 seconds.
Add the broth, Ro-tel, heavy cream, uncooked pasta, and pepper, and give the mixture a good stir. Bring to a boil, cover skillet, and reduce heat to medium-low. Simmer until the pasta is tender, about 15 minutes, stirring once or twice while cooking. Preheat the broiler while the pasta is cooking.
Remove the skillet from the heat, and stir in 1 cup of the cheese. Taste and add salt as needed. Top with the scallions and remaining cheese. Broil until the cheese is melted and lightly browned – about 3 minutes.