Finely dice rhubarb stalks and add to a saucepan with maple syrup. If using, add your additional berries here as well. Cook on medium heat until the rhubarb starts to soften and release liquid, then smash with the back of a spatula to help break down.
Lower the heat to a simmer, add chia seeds, lemon juice and vanilla if using and cook for 15 minutes until mixture thickens, stirring to avoid burning.
Transfer mixture into a jar, let thicken and cool before covering or refrigerating.