Chop the pork into 1-inch cubes and coarsely chop the bacon. Chill in the refrigerator for 20-30 minutes. Meanwhile, sprinkle the gelatin over the cold water or stock and let stand in the refrigerator until needed.
When the meat is thoroughly chilled, place the garlic, rosemary, and salt in a food processor and pulse until the garlic is very finely chopped. With the motor running, add the meat and fats to the bowl a few pieces at a time. (Work quickly to keep your processor from overheating.) Grind until they are a smooth paste, then pour in the gelatin mixture and process until the mixture is once again smooth.
Shape the paste into a 3-4″ diameter log and place at one end of the cheesecloth. Lay extra sprigs of rosemary all around the log, if desired. Roll the cheesecloth up as tightly as possible and tie the ends with kitchen twine. Place in the refrigerator for at least 30 minutes and up to 24 hours.
When you’re ready to cook the pork log, preheat the oven to 375 F. Place the meat in a baking pan and create a bain marie by pouring in boiling water to at least halfway submerge the meat. Bake for approximately 1 hour and 15 minutes (give or take 15 minutes depending on the thickness of the log), until the meat registers 155 F on a meat thermometer.
Cool completely, then using a very sharp knife, shave it into very thin slices.