If using large berries, like strawberries, coarsely chop them, place in small saucepan. Cut up plums to smallish dice, add to saucepan alongside blueberries or other berries of choice. Place over medium heat and bring to a simmer. Allow to simmer, stirring occasionally, until berries are softened, about 5 minutes.
Gently mash berries using a potato masher or fork, leaving a few chunks if you like. Stir in balsamic vinegar, stevia, chia seeds and salt and bring back to a simmer. Allow to simmer, stirring occasionally, until thick and syrupy, about 10 minutes. Remove from heat and allow to cool.
Transfer to sterilize kiln jar or jam jar. Store in the refrigerator for up to 2-3 weeks.