Toss the chicken and onion in a big pot.

Add enough water to cover the bones.

Simmer over low heat, partially covered until reduced to about two cups. This takes around 6-8 hours. Keep an eye on it. The last thing you want is for it to go dry and burn.

Strain through a colander to catch all the bones.

Strain again through a fine mesh strainer to catch any small bits.

To store, pour 1/2 cup into a medium ziploc freezer bag, squeeze the air out and freeze. Place a cookie sheet into the freezer as flat as you can get it and put the ziploc bags onto the cookie sheet to freeze up. An ice cube tray is another approach.

Concentrated chicken stock keeps a month or two in the freezer.

When you need some extra wow just break off a piece and melt it into the sauce.