Prepare peppers- halve them and remove seeds and membranes, it's a good idea to wear gloves when doing this.
Brown the sausage with the onion and garlic, breaking the meat up finely while cooking. Pour off all the grease.
Add the cream cheese and parmesan cheese, stirring well. You want this completely mixed.
Stuff the peppers with the sausage mixture and bake in a 400°f oven for 20 minutes. Serve warm, although they're also good at room temperature.