Preheat your oven to 375°F. Line a 12-cup muffin pan with paper liners or grease the cups well.
For the crumble topping, in a small bowl stir together the ½ cup flour, ⅓ cup sugar, and 3 tablespoon melted butter until crumbly. Set it aside while you make the muffin batter. In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, cornstarch, and lemon zest.
In a separate bowl, whisk together the melted butter, eggs, sourdough starter discard, sour cream or Greek yogurt, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula just until combined. Be careful not to overmix.
Fold in the blueberries.
Divide the batter evenly between the 12 muffin cups. I like to fill them almost to the top for a nice domed muffin. Sprinkle the crumb topping evenly over each one.
Bake for 20 to 24 minutes, or until the tops are lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the muffins rest in the pan for about 5 minutes, then transfer them to a cooling rack.