Pork loin is pretty lean, there is usually a 'cap" of fat on the top and can be taken off if you really feel that you don't want it on there. Although the little added fat will help it stay moist when cooking. Usually fresh herbs and spices will always be better than any dried or ground version but if all you have are dried spices it should be fine. You could always just increase the amount you use to season if the seasoning last time was too weak. Onions are a aromatic so yes they would add some flavor. You can serve pork loin with basically anything because of it's mild flavor and lean cut so think of it like pairing chicken breast.