Combine apple and/or pear slices and lemon juice in mixing bowl. Combine flour, brown sugar, and cinnamon. Add to fruit mixture, toss to coat. Set aside.
Roll out puff pastry into a 14-inch square. Gently ease rolled pastry into a 10- to 11-inch tart pan, lightly pressing pastry to sides of tart pan and leaving remaining pastry hanging over edge of pan. Brush bottom with beaten egg white. Transfer fruit into pastry-lined tart pan. Dot with butter or margarine. Fold edges of pastry over toward center to partially cover fruit.
Bake in a 350 degree F oven for 50 to 55 minutes or until pastry is golden brown and apples are tender. Serve warm with sweetened whipped cream or ice cream, if desired. Makes 12 servings.