Put the water in the bottom of your sauce pan along with the onion and bring to a simmer with the lid on for 10 minutes.
Strain out any extra liquid and put the steamed onion into a thin kitchen towel or tea towel and fold it up. I like to lay the towel out horizontal and fold the top and bottom up over the onion, then fold the sides in – like making a burrito. Their little chests are so tiny so just be sure the onions in the middle aren’t all spread out.
Let the poultice cool for about 5-10 minutes before applying to your child’s chest. My youngest needs it cooled even longer as her tolerance for warm even in a bath is cool to me! Let the poultice sit on their chest up to 20 minutes if they will sit there long enough.
You can re-heat the poultice throughout the day. I like to just put the whole poultice right back in the pan and warm it on low heat. Discard the onion after using it for a day.