Add olive oil to oven-safe skillet over medium high heat until just smoking. Add sausage and onions and cook until lightly browned, about 4 minutes. Add garlic and cook until fragrant, about 30 seconds
Add broth, tomatoes, cream, pasta, salt and pepper to taste and stir. Bring to a boil, cover skillet, and reduce heat to medium-low. Simmer until pasta is tender, about 15 minutes.
Remove skillet from heat and stir in 1/2 cup cheese. toop with remaining cheese and sprinkle with scallions. Broil until cheese is melted, spotty brown, and bubbly