Grind the diced fat and place in a large skillet. Fry until white pork fat is transparent, stirring occasionally. Add the onion, garlic, and bacon. Sauté until bacon is golden brown and the fat has rendered out. Add the apples and season to taste with pepper and salt. Transfer to a stoneware crock or heatproof jar. Leave at room temperature until fat has solidified. Then store, covered in the refrigerator for up to two weeks. Serve this spread on rye bread with sliced onion on top.