In a medium bowl, mix together ¼ cup white sugar, ¼ cup brown sugar, ½ cup all-purpose flour, and ½ teaspoon cinnamon. Cut in 4 tablespoons of cold, cubed butter until the mixture resembles coarse crumbs. Chill in the refrigerator.

Preheat your oven to 375°F (190°C). Grease a baking pan to prevent sticking.

In a large bowl, whisk together 1.5 cups of all-purpose flour, 2 teaspoons of baking powder, and ½ teaspoon of salt.

Cream together ½ cup butter and 1 cup sugar until pale and fluffy. Add in 1 large egg, mixing until incorporated.

Whisk together 1 cup sourdough starter, ½ cup milk, 1 teaspoon vanilla extract, and the zest of 1 lemon. Pour into the creamed mixture, blending gently.

Gradually mix the dry ingredients into the wet ingredients on low speed until just combined.

Toss 1 cup fresh blueberries with 1 tablespoon flour to coat. This helps prevent sinking.

Spread half of the batter in the prepared pan, sprinkle with half blueberries, dollop and spread remaining batter, scattering the rest of the blueberries on top.

Distribute the chilled crumble topping over the batter.

Bake for 45-50 minutes, until golden brown and a toothpick comes out clean.

Let cool in the pan for 10-15 minutes before transferring to a wire rack. Slice and serve warm or at room temperature.