Rinse six 1-cup glass containers and lids with boiling water. Dry thoroughly. Set aside.
Add mashed peaches, sugar, apple, lemon juice, and butter to a medium saucepan. Cook over medium-high, stirring frequently, until jam is thickened, about 25 minutes. The butter in the recipe helps reduce foam formation. If you see any foam appear, skim off with a spoon.
Carefully, transfer jam to prepared jars and let cool at room temperature. Cover the jars. Store in the refrigerator up to 2 weeks or freeze up to 1 year.
Apples contain pectin, a soluble fiber that gives jams and jellies thickness and body.