When you have an abundance of juicy sweet Concord grapes, this superb jelly is quick to make and sings of grape flavor.
Put the grapes and apple in a large pot and add 1/2 cup of water. Bring to a boil, cover and simmer over low heat until very juicy, about 10 minutes. Strain the juice through a fine sieve set over a large bowl.
Strain the juice again through several layers of dampened cheesecloth, without pressing or squeezing; you should have 3 cups.
In a very large saucepan, boil the grape juice and sugar over moderate heat, stirring, until the jelly falls off a cool spoon in sheets and registers 220° on an instant-read thermometer, about 18 minutes.
Pour the jelly into hot, sterilized canning jars to within 1/4 inch of the rims. Seal. Submerge the jars in a pot of water and boil for 5 minutes. Carefully remove the jars and let cool. Check the jar lids to make sure they’ve fully sealed; refrigerate any jars with imperfect seals.