Place pork fat cubes in a large heavy pan or Dutch oven over low heat and slowly pan-fry until the cubes render their lard and become browned and crisp, about 30 minutes. Stir occasionally. Remove the pork cracklings with a slotted spoon and drain on paper towels.
Stir ground pork, onions, salt, allspice, and black pepper in the Dutch oven with rendered fat and slowly pour in 2 cups boiling water; bring to a boil, reduce heat to low, and simmer with the pan partially covered until water has evaporated, about 1 1/2 hours.
Place browned crackling cubes into a food grinder and coarsely grind; thoroughly stir the ground cracklings into the pork mixture. Simmer the mixture for 30 more minutes uncovered. Remove from heat and allow to cool until mixture can be handled, about 20 minutes. Grind the cretons mixture with the food grinder.
Spoon about 1/4 cup of the mixture into small jars or lidded containers and cover with plastic wrap; push the wrap down onto the surface of the cretons to completely cover. Cover containers with lids and refrigerate or freeze until serving time. Let cretons come to room temperature before serving.