Place frozen berries and maple syrup in a saucepan and cook over medium high heat until they start to break down and release juices. Stir frequently. Once berries have softened and started to break down, mash them with the back of a fork. Remove berries from heat and stir in chia seeds and lemon juice. Let sit for about 10 minutes to cool and thicken.
Preheat oven to 350 degrees. Prepare a square baking dish (8x8) with cooking spray, butter or oil. Mix dry ingredients together in a large bowl: oats, baking powder, cinnamon, salt. NOTE: if you are using old fashioned oats, you might want to pulse them in a food processor or blender a few times to make them more the consistency of quick oats. In a separate bowl, mix together wet ingredients except for peanut butter: banana, egg, butter, maple syrup, milk, vanilla. Add the wet ingredients to the dry ingredient bowl and stir until combined. Add the peanut butter and stir until incorporated. Pour oatmeal mixture into prepared baking dish. Dollop spoonfuls of the chia jam over the oatmeal and then use a knife to swirl in. Bake at 350 for 25-30 minutes, or until it’s starting to pull away at the edges and lightly brown and is set in the middle.