Preheat the oven to 350°F (180°C) and line an 8 x 8-inch (20 x 20cm) square baking dish with parchment paper.
In a small bowl, combine the ground flaxseed with the water. Leave for 5 minutes until thick and gelatinous.
In a large mixing bowl, mash the banana. Add the flaxseed mixture, maple syrup, plant-based milk, and vanilla extract. Mix to combine. Add the rolled oats, baking powder, cinnamon, and salt. Mix again until combined.
Transfer the mixture to the lined baking dish and press down evenly with the back of a spoon. Bake for 15 minutes.
Add the berries to a small pot and simmer over medium heat for 3–4 minutes until they start to release their juices. Remove from the heat and stir in the chia seeds.
Wipe out the mixing bowl and add all crumble topping ingredients. Mix until combined. The texture should be dry and coarse.
Remove the base from the oven. Spread the berry mixture on top, followed by the crumble topping. Bake for 10 minutes or until the crumble is lightly golden.
Leave to cool in the pan for 15 minutes before slicing into 12 bars. Store in an airtight container in the refrigerator for up to five days.