Put the pig's feet in a heavy pot and add the quartered onions and the bouquet garni.

Pour in enough cold water to cover the contents well.

Season with salt and pepper.

Over high heat bring the liquid to a boil, skimming it.

Then reduce the heat, partially cover the pot, and simmer the contents for five hours, or until the bones separate easily from the meat and the cooking liquid is reduced by half.

Remove the pig's feet from the liquid with a skimmer, bone them and place them in a large bowl.

Strain the cooking liquid and pour it over them.

Refrigerate the pig's feet overnight.

The next day, melt the butter in a saucepan over medium heat.

Add the diced onions and stir with a wooden spoon until they are soft and golden about 10 minutes.

Lift the pig's feet out of the jellied cooking liquid and lay them on top of the onions, then spoon enough jelly over them to barely cover the pig's feet.

Partially cover the pan and simmer the contents for one hour to reduce the liquid to a thick gravy.

Serve the pig's feet and gravy from a warmed dish and accompany them with a pot of mustard.