In a small saucepan over medium heat, add frozen berries and heat through for about 5 minutes. The berries will break down and begin to bubble, stir occasionally. Use a fork or potato masher to mash the berries to your desired consistency. (Leaving some chucks provide a nice texture to the jam)
Add the remaining ingredients: lemon, maple syrup, chia seeds, and vanilla. Mix well and let the mixture cook for about 5 minutes, stirring occasionally.
Remove the jam from the heat and let it cool for about 10 minutes (the longer the better) before using. The jam will thicken as it cools.
Store in a sealed jar in the fridge for up to 3 weeks.