Heat oven to 375°F. In medium bowl, mix flour, 1/2 cup sugar, the baking powder and salt. Stir in sour cream, butter and egg until well blended; set aside.
In 2-quart saucepan, mix 3/4 cup sugar, the cornstarch and reserved raspberry liquid. Cook over medium heat, stirring occasionally, until mixture boils. Boil 1 minute, stirring constantly. Stir in raspberries and peaches. Cook 1 minute, stirring constantly. Pour into ungreased 2-quart casserole.
Spoon biscuit mixture over hot fruit mixture, forming 9 biscuits around edge of casserole. Sprinkle biscuits with 1 tablespoon sugar.
Bake 35 to 45 minutes or until biscuits are golden brown. Serve warm.