Wash and prep the rhubarb and peaches. Peel peaches by blanching if desired.
In a large bowl, mix rhubarb, peaches, sugar, and lemon juice. Let macerate 1 hour.
Pour mixture into a large saucepan and bring to a boil over medium heat.
Reduce to a simmer. Cook uncovered, stirring often, for 30–40 minutes.
Use the freezer plate test to check for doneness. Jam should wrinkle when touched.
Remove from heat and ladle into sterilized jars, leaving ¼" headspace.
Seal and refrigerate, freeze, or water-bath can for shelf-stable storage.
Let jars cool undisturbed for 12 hours. Store in a cool, dark place.
Notes
This jam can be frozen for up to 12 months or canned using a water bath. Add vanilla, ginger, or even strawberries for fun variations. Try it on biscuits, yogurt, or cheese boards.