Add raspberries to a medium saucepan over medium heat. The berries will start to release their liquid and break down after a few minutes. Use a potato masher to gently break down larger pieces, but don't mash too much, you want to leave some texture. Bring to a simmer for 3-5 minutes until it’s slightly thickened and saucy. Stir frequently.
Stir in the agave, lemon juice, ground chia seed, and salt. Cook on low for another 1-2 minutes to thicken (it'll thicken a lot more when it cools). Taste for flavor and add more if needed.
Remove from heat and let it completely cool. Pour into an air-tight jar and store in the fridge for 5-7 days.
Notes
Chia Seeds - if you don't have ground, you can use a coffee grinder or you can use the seeds whole (just double the amount). The whole chia seeds will add more texture, but they will soften perfectly in the jam.
Other Berries for Jam - strawberries, blueberries, lingonberry, blackberries, gooseberries, or, cherries, cranberries. If you use fresh strawberries, trim stems and chop them before measuring. For frozen strawberries, thaw first before using.
Flavor Add-In Ideas - ½ teaspoon vanilla extract, a pinch of nutmeg or cardamom, 1 teaspoon of lemon zest or orange zest, ½ teaspoon fresh minced ginger, 1-2 teaspoons fresh-chopped mint, or basil.