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