Heat oil over medium-high heat in an oven safe skillet until hot. Add the sausage and onion and cook until lightly browned, 4-5 minutes. Add in the garlic and cook until fragrant, 30 seconds.
Add the broth, rotel, heavy cream, uncooked pasta, and pepper. Stir well. Bring to a boil, then cover and reduce heat to medium-low. Simmer until the pasta is tender, 15 minutes. Stir once or twice while cooking. Preheat the broiler while pasta is cooking.
Remove the skillet from the heat, 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.