Preheat oven to 425 degrees F. Blend or process frozen strawberries and jelly until smooth. Stir in sliced berries; cover and refrigerate.
Roll pastry to 12 x 10 inches; cut in eight rectangles. Place pastry rectangles 1 inch apart on large baking sheet; pierce with fork tines. Cover with second baking sheet. Bake 6 minutes. Remove top baking sheet; bake 4 minutes more. Remove from oven; let stand on sheet. Reduce oven to 325 degrees F.
For meringue, in small saucepan cook and stir 1/2 cup water and the cornstarch over medium heat until thickened and bubbly. Remove from heat; cool slightly. In bowl combine sugar and cream of tartar; set aside. In large mixing bowl beat egg whites and vanilla on medium to high until frothy. Gradually beat in sugar mixture until soft peaks form. Add cornstarch mixture; beat to stiff peaks. Spoon on pastry; bake meringue 15 minutes. Serve pastries warm with sauce; top with fresh whole strawberry. Serves 8.