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 until well combined. Let the mixture stand for 5 minutes to allow pectin to activate.
03 - Place the pot over medium-high heat and bring the 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 hard for 1-2 minutes to reach setting point.
05 - Remove from heat and skim off any foam that has accumulated on the surface using a metal spoon.
06 - Carefully 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 freshness.