For pastry, in a mixing bowl combine the flour and salt. Cut in the chilled margarine until pieces are the size of small peas.
Sprinkle 1 tablespoon of the cold water over part of the mixture. Toss with a fork. Push to the side of the bowl. Repeat until all of the mixture is moistened. Form into a ball.
On a lightly floured surface roll pastry into a 12-inch circle.
Wrap pastry around the rolling pin. Ease into an 11-inch flan pan. Do not stretch. Press pastry up the sides of the pan. Trim pastry even with the top of the pan. Prick bottom of crust well with tines of a fork.
Bake in 450 degree F. oven for 10 to 12 minutes or until golden. Cool on wire rack.
Meanwhile, in a small mixing bowl stir together cream cheese, sugar, and vanilla until smooth. Spread atop cooled crust. Arrange peach slices in circles atop cheese mixture.
In a small saucepan heat apple jelly until melted. Spoon atop peaches. Chill for at least 2 hours or up to 8 hours. Makes 12 servings.