Cook the mixed berries in a saucepan over medium heat until they start breaking down, about 5-7 minutes. Mash them slightly with a fork or potato masher until you reach your desired consistency.
Mix in chia seeds, and honey (or maple syrup). Cook for another 2 minutes.
Stir in the lemon juice.
Remove from heat and let it sit for about 10 minutes. The chia seeds will absorb the liquid and naturally thicken the jam.
Transfer to a clean jar, let it cool completely, then keep it refrigerated for up to a week.