Working in batches, place the apples, plums and onions into the work bowl of a food processor, and grind until fine. Transfer the ground fruit and onions into a large pot, and stir in the malt vinegar, sugar, salt, Worcestershire sauce, garlic, ginger, nutmeg, allspice, and cayenne pepper until the mixture is well combined. Bring to a simmer over low heat, and simmer until brown and thickened, about 3 to 4 hours.
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the sauce into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot, and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 2 inch space between the jars. Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.