In a bowl, combine the flour, sugar and salt; cut in the shortening until crumbly. Gradually add water, tossing with a fork until a ball forms. Cover and refrigerate for at least 1-1/2 hours.
On a lightly floured surface, roll pastry into a 9-in. circle. Transfer to a foil-lined 15x10x1-in. baking pan.
In a bowl, combine the plums, 3 tablespoons sugar and flour. Place in the center of pastry. Bring edges of pastry over filling, leaving 3-1/2 in. of filling uncovered. Brush pastry with egg white, then sprinkle with remaining sugar.
Bake at 375° for 40-45 minutes or until bubbly and crust is golden brown.