Crumble sausage into 10" saute pan set to medium high heat. Peel and chop onion into 1/4" dice, and add to sausage as it cooks.
When sausage is cooked through, about 5 minutes, add the uncooked pasta to the pan, and stir well.
Add broth to pan, making sure that the sausage and pasta is covered by the liquid by at least a 1/2 inch. Cover pan, and bring to rolling boil over high heat. After mixture boils, remove lid and turn heat to low.
Continue cooking for 12 to 15 minutes until pasta is almost cooked through. There should be some liquid left in the pan. If the pan looks dry at any time, add more broth, 1/2 a cup at a time.
Add spinach and stir well. If there is too much liquid remaining, turn heat to high for a few minutes to allow to evaporate to desired consistency. When spinach is cooked through, and the sauce is to the thickness desired, remove from heat and serve.
Sprinkle with your favorite cheese and red pepper flakes to taste.