Sugar, as needed
2 tbsp. lemon juice
2 1"-wide strips lemon zest
1. Put apples and 6 cups water into a large heavy pot and bring to a boil. Reduce heat to medium-low and simmer, covered, until apples are broken down and pulpy, about 25 minutes. Set 6–8 layers of cheesecloth over a medium bowl, allowing cheesecloth to hang generously over sides. Transfer apples with their juice to center of cheesecloth and draw up sides to form a "purse". Cinch closed with kitchen twine, then tie to a long wooden spoon or dowel. Set the bowl inside a large stockpot. Rest wooden spoon across top of stockpot, letting purse hang freely inside. Strain through a fine sieve any juice that immediately collects in the bowl and transfer juice to a large measuring cup; cover with plastic wrap and refrigerate. Allow apples to continue to strain overnight.
2. Put a large plate into freezer to let chill. Discard apple purse. Strain accumulated juice through a fine sieve into the same measuring cup with the refrigerated juice. You should end up with about 6 cups of juice. For every 2 cups of juice, measure 1 lb. of sugar (about 2 1⁄2 cups). Combine strained juice, sugar, lemon juice, and lemon zest in the large pot and bring to a boil, stirring occasionally, over medium-high heat. Boil vigorously, skimming off and discarding any foam that rises to the surface, 18–20 minutes. Test jelly by running a finger through a spoonful of it set on the chilled plate. If the line stays clear—that is, the jelly doesn't ooze back into place—it's ready. Discard zest.
3. Remove jelly from heat and pour into sterilized canning jars. Secure jars with lids and ring bands. Transfer filled jars to a canning rack, submerge in a pot of gently boiling water (jars should be covered by at least 1" of water), and boil for 15 minutes. Carefully lift the jars from water with jar tongs and place on a dish towel to let cool for 24 hours. To test that the jars have sealed properly, press on center of each lid and remove your finger. If lid stays down, it's sealed. Refrigerate any jelly that hasn't sealed and use within 4 weeks. (Sealed jelly will keep indefinitely.) Serve with ham, if you like.
MAKES ABOUT 3 PINTS