Add the milk, Greek yogurt, chia seeds, vanilla extract, and salt to a medium bowl. Whisk until thoroughly combined.
Let the mixture sit for 5 minutes, then whisk it again to break up any clumps of chia seeds.
Divide the chia pudding evenly among four jars or meal-prep containers. Cover and refrigerate for at least 4 hours or overnight.
To make the jammy fruit, add the diced peaches, blueberries, maple syrup, lemon juice, and cinnamon to a small saucepan.
Cook over medium-low heat for 8 to 10 minutes, stirring occasionally, until the peaches soften and the blueberries begin to burst.
Gently mash some of the blueberries with the back of a spoon while leaving pieces of peach intact for texture.
Remove the fruit from the heat and allow it to cool completely. The mixture will continue to thicken as it cools.
Spoon the cooled peach and blueberry mixture over the chilled chia pudding.
Add nuts, seeds, or additional fresh fruit immediately before serving.