01 - Combine strawberries, rhubarb, raspberries, and mixed berries in a large non-reactive pot. Mash lightly with a potato masher to release natural juices.
02 - Stir in lemon juice and fruit pectin powder until well combined. Let mixture stand for 5 minutes to activate the pectin.
03 - Place pot over medium-high heat and bring mixture to a boil, stirring frequently to prevent scorching.
04 - Add all sugar at once and stir until completely dissolved. Return to a full rolling boil and cook vigorously for 1 to 2 minutes to reach setting point.
05 - Remove from heat and skim off any foam from surface using a spoon. Meanwhile, ensure sterilized jars are ready.
06 - Ladle hot jam into sterilized jars leaving ¼-inch headspace. Wipe rims clean, seal with lids, and process in boiling water bath for 10 minutes for long-term storage.
07 - Allow jars to cool completely at room temperature. Refrigerate opened jam and consume within 3 weeks for optimal quality.