Heat oven to 350 degree F. Coat a 6-cup oval baking dish with nonstick cooking spray.
In a large bowl, gently mix blackberries, blueberries, raspberries, sugar, walnuts, orange liqueur and tapioca. Spoon into prepared dish.
In a large bowl, whisk flour, 1/4 cup of the sugar, the baking soda and salt. Mix together milk, sour cream and butter. Add to the flour mixture and stir until a batter forms.
Dollop batter over the fruit. Sprinkle batter with remaining 1 tablespoon sugar. Bake at 350 degree F for 35 to 40 minutes, until nicely browned.
Serve warm or at room temperature with whipped cream, if desired.