Add chopped apples, lemon juice, and sliced lemon. Boil mixture until it reaches the jelly stage (220 degrees), about 15-20 minutes, skimming any foam.
Fill jars leaving 1/4 inch headspace, wipe rims, seal, and then process in a water bath canner for 10 minutes. Turn off burner, remove lid, and let jars sit in canner for 5 minutes before removing to a towel-lined surface.
Let jars cool for 12 hours, remove rings, check seals, label and store in cool, dark place. Use in 12-18 months.