Take the pork fat straight from the fridge, remove any rind, and cut it into 1 cm / 1/2-inch strips, then cubes. Peel the garlic and carrot, and cut the carrot and celery into 5 mm / 1/4-inch dice. Strip the rosemary and sage leaves, pick the parsley, and pat all the herbs dry.
On a 40 x 30 cm / 16 x 12-inch cutting board, chop the fat, garlic, carrot, and celery together with a 20 cm / 8-inch chef's knife for 6 to 8 minutes, regathering the pile as you go, until the pieces are the size of short-grain rice. Add the parsley, sage, and rosemary and chop for 2 minutes more.
Transfer the mixture to a mortar at least 15 cm / 6-inch wide, working in two batches. Add the salt, pepper, and nutmeg, then pound and press for about 5 minutes until it holds together as a coarse paste with visible grains.
Pack the paste into a 350 ml / 12-oz glass jar, pressing it down with the back of a spoon to remove air pockets. Seal and refrigerate for 4 hours so the salt draws the garlic and herb flavor into the fat.