Rinse the liver with cold water, then place it in a bowl of cold water to soak for 30 minutes. This helps remove any bitterness and strong unwanted flavors.
Soak the sausage casing for 30 minutes.
Bring water to a boil in a pot, then add the oats and cook for 30 minutes.
Peel and cut onion into pieces that fit into your meat grinder.
Place the mace, allspice, salt and pepper in a spice grinder and grind until fine.
Once the oats are cooked, pour them onto a plate to cool.
Use paper towel to dry the veal liver.
Cut the liver into smaller pieces and place in a food processor.
Using a chilled grinder, place the pre cut strips of cold beef fat into the feeder tube and grind.
Now grind the onion.
Put the puréed liver, beef fat, ground onion, cooked oats and pulverized spices and seasonings into a large bowl and mix well.
Remove the soaking sausage casing, fold the open end over your hand, like a piping bag and start to fill the casing with the pudding mixture.
Push the sausage into the casing, try to avoid creating air pockets. Leave about 1 inch of casing at the end, twist the casing, then using a piece of cord, tie the end of the casing.
The Black Pudding will sag.
Cook the blood pudding sausage in a large pan / pot filled with water. Maintain the water temperature just under boiling point at 175 degrees F / 80 degrees C. Gently simmer the Black Pudding for 60 minutes. DO NOT pierce the sausage casing with a digital thermometer to check the internal temperature. Only test the water temperature.
After 60 minutes, remove the Black Pudding from the water, dry the casing and allow to cool. The sausage will have firmed up.
Notes
Using veal liver will produce a milder, lighter colored Black Pudding.