Put the liver and pork in a kettle and add the onion and water to cover. Bring to the boil. Cover and cook about 2 hours.
Preheat the oven to 275 degrees.
Remove the meats and onion from the kettle. There should be 2cups of broth. If there is more, skim off the clearest top portion, discarding the bottom portion with its meat residue.
Cut the meats into small chunks. Using a meat grinder or food processor, coarsely grind the meats and onion. Spoon and scrape into a mixing bowl. Stir in the 2cups reserved broth. The mixture will be quite thin.
Stir in the salt, pepper, and sage. Mix well and pour into a heavy tin loaf pan or 2-quart casserole. Place in the oven and bake 2½ hours. The long cooking helps to develop the flavor of the pudding and makes it easy to slice. When cool, place in a cold place or in the refrigerator.