Preheat the oven to 350 degrees. Line a muffin tin with silicone cupcake wrappers OR parchment wrappers.
In a large bowl prepare flax egg by whisking flax and warm water together. Set aside to gel for 10 minutes.
Once thickened, whisk in mashed banana, oil, maple syrup, vanilla and sourdough discard.
Next add in dry ingredients. Switch to a spatula or a spoon to mix thoroughly.
Finally, gently fold in wild blueberries if using and scoop into muffin liners. IF USING JAM METHOD, spoon a heaping tbsp into the bottom of cups, then add 1 tsp of blueberry chia jam in the middle and top with remaining batter.
In a separate medium sized bowl mix together the crumble topping. Use your fingers to top each muffin. Place in a preheated oven and bake for 20-22 minutes or until the toothpick comes out clean. Do not overbake or they become too dry.
Once done, remove from the oven and let rest in the muffin tin on a wire rack for 10 minutes. Gently remove from tin and allow to cool completely before eating.