For filling, in a medium saucepan, stir together rhubarb, strawberries, the 1/2 cup granulated sugar, and the 1/4 cup cold water. Cook, covered, about 5 minutes or until rhubarb is tender. Stir together the 2 tablespoons water and the cornstarch. Stir into rhubarb mixture. Cook and stir until thickened and bubbly. Set aside to cool.