Lemon Blueberry Coffee Cake

Prepare the Blueberry Filling: In a saucepan over medium heat, combine blueberries, 3 tablespoons white sugar, and 1 tablespoon lemon juice. Cook, stirring frequently, until blueberries release their juices. Mash blueberries with a potato masher until broken up. Bring to a simmer, then reduce heat to low.

Thicken the Filling: In a small bowl, whisk together cornstarch and water. Add to the blueberry mixture, stirring constantly. Cook until thickened, about 1-2 minutes. Pour into a shallow bowl and refrigerate to cool.

Make the Crumb Topping: In a food processor, combine 1/2 cup flour, brown sugar, 1/8 teaspoon salt, and 1/8 teaspoon nutmeg. Pulse a few times to combine. Add 1/4 cup cold butter and pulse until the mixture resembles coarse sand. Pour into a small bowl. Press the mixture together with your hands to form medium to large crumbs. Refrigerate.

Preheat Oven: Preheat oven to 350 degrees F (175 degrees C). Line an 8x8-inch square pan with parchment paper, leaving overhang on all sides.

Prepare the Cake Batter: To the food processor (no need to clean), add 1 1/2 cups flour, 1/2 cup white sugar, baking powder, baking soda, 1/2 teaspoon salt, and 1/4 teaspoon ground nutmeg. Pulse a few times to combine. Add 1/2 cup cold butter and pulse until the mixture resembles coarse crumbs.

Combine Wet Ingredients: In a small bowl or measuring cup, whisk together buttermilk, egg, lemon zest, lemon juice, and vanilla. Add this mixture to the food processor and pulse until just combined, scraping down the sides as needed. The batter may appear lumpy from the cold butter.

Assemble the Cake: Pour about 2/3 of the batter into the prepared pan, smoothing into an even layer. Remove blueberry filling and spread evenly over the batter. Dollop the remaining batter over the blueberry filling, spreading until most of the filling is covered. Remove crumb topping and sprinkle evenly over the top, lightly pressing to adhere.

Bake: Bake in the preheated oven for 45-55 minutes, or until the middle springs back lightly when touched. Cool in the pan for about 10 minutes. Use the parchment overhang to remove to a wire rack to cool completely, about 20 minutes. Cut into 12 pieces.