Preheat the oven to 350 degrees. Line a muffin tin with cupcake liners.
In a medium bowl combine the flour, baking soda, and sugar. Set aside.
In a large bowl mix together the sourdough starter, egg, butter, vanilla, and lemon juice. Mix well.
Add the flour to the butter mixture and mix until just combined.
Fold in the blueberries and blackberries.
Divide the batter between the muffin cups filling about 2/3 of the way full.
Mix together all the ingredients for the topping and mix until a crumble forms. Sprinkle 1-2 teaspoons of the crumble on top of each muffin.
Bake for 20 minutes.
Remove from oven and cool completely on a wire rack.