Thinly slice onion and cook in oil to a translucent look approximately 7-8 minutes. Don't caramelize the onion.
Sprinkle flour over the onions to soak up the oil.
Using a flour sack or old t-shirt, make an onion omelet by placing the onion mixture into the cloth.
Check the temperature of the onion poultice on your arm before placing it on the patient's chest. It needs to be as warm as the patient can handle.
Leave it on the patient for 30 minutes.