Add olive oil to an oven-safe skillet over medium high heat until just smoking. Add sausage & onions. Cook until lightly browned, about 5 minutes. Add garlic & cook until fragrant, about 1 minute.
Add broth, tomatoes, green chiles, milk & penne to the skillet. Bring to a boil, cover & reduce heat to medium-low. Simmer until pasta is tender & liquid is absorbed, about 15 minutes.
Remove skillet from heat & stir in 1 cup of cheese & half the green onions. (If you didn't use an oven safe skillet, place contents into a 13x9" baking dish now.) Top with remaining ½ cup of cheese & other half of the green onions. Broil until cheese is melted, & starts to brown.
Notes
*Can substitute the diced tomatoes and green chiles with 1 can of Ro*Tel