In a saucepan over medium heat, add chopped peaches, granulated sugar, lemon juice, and cornstarch. Stir to combine (if needed, add a splash of water to get the right consistency). Bring to a simmer, then reduce heat to medium-low. Simmer for 10 minutes, stirring occasionally.
Remove from heat and gently mash peaches with a potato masher until desired consistency is reached. Return to heat and cook for another 2-4 minutes, or until mixture has reduced slightly. The mixture should thickly coat the back of a spoon (and will continue to thicken as it cools).
Remove from heat and transfer to jars, leaving about ½" of space at the top of each jar (jam will expand slightly when frozen). Allow to cool completely before adding lids. Store for up to 1 year in freezer, or 3-4 weeks in the refrigerator.
16 ounces diced peaches = about 4 medium peaches with stems, pits, and skins removed.
Freezer jam is not shelf stable and needs to be stored in the refrigerator or freezer at all times. Jam will keep in the refrigerator for 3-4 weeks in a tightly sealed jar, or in the freezer for up to a year.
Sugar can be substituted with a sweetener alternative, but please consult your sweetener package for proper ratios (some are more concentrated than others).
If using a sugar substitute, keep in mind that regular sugar preserves the jam, so sugar-free jam will not keep as long in the refrigerator.