Cut the ends of the onions, remove the skins and cut into halves or fourths.
Rinse themeat and remove anything undesirable
Put all ingredients in a large stock pot with water that does not totally cover all the meat and simmer for about six hours. Goal is to make the meat fall of the bones and so tender it falls to pieces at a mere touch, and turn the onion to mush.
Take stock pot off the heat and let it cool for a bit. If you let it cool overnight, put it someplace where the temp is below 6 deg C (42F), and then warm it back upthe next day. The meat should be hot but cool enough to handle.
Ladle out several cups of the broth into a container and cool it until the fat has collected at the top and hardened. Dispose of the hardened fat. An hour or two in the freezer should do the trick.
Once the meat has cooled enough so that you can work with it (still very warm but cool enough to handle) fish the meat and onion out of the pot. Clean the meat of all the bones. Make sure you catch all the little bones so that nothing but softmeat remains. Clean off any excess fat still left on the meat as well as anyother undesirables.
Put the clean meat and the onions into a food processor, and puree until consistent .You'll likely need to do this in small batches and put each batch into a large mixing bowl. Overloading the food processor will make it hard to get the right consistency.
Once everything has been fished out of the pot discard the remaining broth (keeping the several cups of broth you set aside earlier) and clean the pot. Put the mixture back into the pot and warm it up over a low heat while you mix it thoroughly. Once you have heated the mixture back up to a point where the fat and other juices have melted and are well mixed, take the cold broth you set aside and tossed the fat from, and mix little by little back into the mixture until you have desired consistency. For the optimal spreadability the mixture should be somewhat wet.
Take the pot of the stove and let it cool down, stir it occasionally as it cools down to ensure the broth stays mixed in with the mixture.
Once cool down, put into small container and freeze. Due to lack of preservatives each container should be consumed in a about 3-4 days. I like 70ml (0.3 cups) Ikea PRUTA containers for this.