Add olive oil to an oven-safe skillet, over medium high heat, until just smoking.
Add sausage and onions; cook until lightly browned, about 4 minutes.
Add garlic and cook until fragrant, about 30 seconds.
Add broth, tomatoes, cream, pasta, salt and pepper 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.
Top with remaining cheese and sprinkle with scallions.
Broil until cheese is melted, spotty brown, and bubbly.
