01 - Combine strawberries, rhubarb, raspberries, and mixed berries in a large non-reactive pot. Mash lightly with a potato masher to release juices.
02 - Stir in lemon juice and fruit pectin. Mix thoroughly and let stand for 5 minutes to activate pectin.
03 - Place pot over medium-high heat. Bring mixture to a boil, stirring frequently to prevent scorching.
04 - Add sugar all at once, stirring until completely dissolved. Return to full rolling boil and cook hard for 1-2 minutes.
05 - Remove from heat. Skim off foam with a spoon. Carefully ladle hot jam into sterilized jars, leaving ¼-inch headspace.
06 - Wipe jar rims clean, seal with lids, and process in boiling water bath for 10 minutes for long-term storage.
07 - Allow jars to cool completely. Refrigerate opened jam and use within 3 weeks.