1 cup (250 mL) all-purpose flour
½ cup (125 mL) packed brown sugar
¼ tsp (1 mL) salt
½ cup (125 mL) cold unsalted butter
½ cup (125 mL) room temperature unsalted butter
2 cup (500 mL) all-purpose flour
1 ½ tsp (7 mL) baking powder
½ tsp (2 mL) salt
¾ cup (175 mL) granulated sugar
1 large egg
1 tsp (5 mL) pure vanilla
½ cup (125 mL) milk
5 cup (1250 mL) fresh blueberries
½ cup cooked caramelized Vidalia onion (optional)
reserved Streusel Topping
1. Using a fork, mash ingredients until crumbly. Set aside.
1. Sift together flour, baking powder, and salt. Set aside.
2. In the bowl of a stand mixer fitted with paddle attachment, cream together butter and sugar on medium speed until light and fluffy, about 3 minutes. Reduce speed to low and beat in egg and vanilla.
3. Add reserved flour mixture, alternating with milk and ending with flour mixture, then gently fold in blueberries and, if using, caramelized onions.
4. Spread batter into a greased and floured 10-inch spring form pan, top with streusel mixture and bake on centre rack of a preheated 350 degrees Fahrenheit oven (180 degrees Celsius) until a toothpick inserted in the centre of cake comes out clean, about 60 minutes.
5. Transfer buckle to a wire rack and cool for 10 minutes before removing from pan, then let cake cool for another 15 minutes before serving.