Chill all the ingredients beforehand to make them easier to work with.
Dice the pork meat and pork belly into pieces so that they will fit into the meat grinder. Cut the onion into smaller pieces and pass it along with the meat through the grinder twice through the coarse setting.
Collect all the ground meat in a bowl and add the spices, egg whites and potato starch. Combine until everything is evenly distributed.
Then, add the chicken stock a little at a time while you stir to incorporate it. The aim here is to make a malleable, soft mixture, but we don't want it to be too runny.
Once you've achieved the right consistency, cover the bowl and place in the fridge to let the mixture rest for 30 to 40 minutes.
Meanwhile, run the casings under cold water, making sure to rinse them well both inside and out. Watch out for any holes that the water might seep out from.
Heat up a little oil in a small frying pan and cook a small piece of the meat mixture. Taste and adjust the seasoning to your preference.
Use a sausage filler (you can find many models on the internet) to fill the sausage casings with your mixture. The traditional way is to make a single long sausage for each casing.
Allow the sausages to rest in the fridge for half an hour before continuing.
Now it's time to cook your delicious sausage! You can do this 2 ways. Either pan-fry the sausage with a dollop of butter, turning it over a few times to cook evenly on both sides (this should take around 15-20 minutes).
Or you can place it in a skillet with a knob of butter and some water, turning it once before the water evaporates and the sausage starts to fry. This method pre-boils the sausage as well, saving you some effort!