Preheat oven to 350°F. Lightly grease a 9×13-inch baking dish and set aside.

Make the Crumb Topping: In a medium bowl, mix flour, brown sugar, granulated sugar, cinnamon, and salt. Stir in the melted butter until evenly combined. Cover and place in the refrigerator.

Prepare the Batter: In a large mixing bowl, whisk together flour, sugar, salt, baking powder, and baking soda. In a separate bowl, whisk eggs, buttermilk, milk, melted butter, lemon zest, and vanilla. Don’t worry if the butter clumps slightly due to the cold ingredients.

Pour the wet ingredients into the dry ingredients. Stir gently with a rubber spatula until just combined. The batter will be lumpy—avoid overmixing.

Pour the batter into the prepared baking dish. Sprinkle blueberries over the top.

Remove the crumb topping from the fridge and crumble it evenly over the batter and berries.

Bake for 35–45 minutes, or until the casserole is puffed and golden, and a toothpick inserted in the center comes out clean.

Serve warm with maple syrup.