Cut the silver membrane from the fillet if there is any and remove any excess fat. With a sharp knife, cut fillet into 1/2 to 3/4 inch slices.
Pour flour into a large Zip Lock bag, and place 4-6 pieces of cut pork in with flour until they are all floured. Gather the top together tightly and shake the medallions well. Slice large onion in 1/8 inc. slices and then halve slices.
Heat 1/2 cup oil in a saute or frying pan with lid. (Bacon grease would be excellent if you keep it on hand.) Shake each piece of pork to remove excess flour and place in hot pan. Sprinkle with salt, pepper, and onion powder. Brown on one side, turn and spice the other side and brown once again. Cover with onions and enough water to almost cover the medallions. Cover with lid and cook until very tender. If gravy isn't thick enough when medallions are tender, add some cornstarch to a bit of water, make a slurry and add to pan.
NOTE: You may need two saute/frying pans depending on how big the fillets are. Make sure they have lids.