This bright-green jelly is traditionally served with lamb.
Put mint leaves in a small bowl and cover with boiling water. Let stand for 1 hour, pressing juice from the leaves every so often. Remove leaves.
In a large kettle over medium heat, combine apple juice and mint extract. Bring to a boil, then add sugar. Continue to boil until mixture reaches 221 degrees on a candy thermometer or falls in sheets from a spoon, 20 to 30 minutes. Stir occasionally.
Remove from heat and discard mint leaves, if added. Add a few drops of green food coloring, if desired. Skim off and discard any foam. Carefully ladle jelly into hot, sterilized jars, leaving 1/4-inch head space. Wipe excess jelly from inside and outside rims, then seal.
For a stronger mint flavor, tie bruised mint leaves in two thicknesses of cheesecloth and boil with the juice.