Add olive oil to a 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 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 ½ cup cheese.
Top with remaining cheese and sprinkle with scallions. Broil until cheese is melted, spotty brown, and bubbly.