In a large bowl, combine the flour, salt and baking powder. Cut in shortening until mixture resembles coarse crumbs. In a small bowl, whisk egg yolks, lemon juice and water; gradually add to flour mixture, tossing with a fork until dough forms a ball. Divide in half, making one portion slightly larger. Chill for 30 minutes.
Roll out larger portion of dough between two large sheets of waxed paper into a 17x12-in. rectangle. Transfer to an ungreased 15x10x1-in. baking pan. Press pastry onto the bottom and up the sides of pan; trim pastry even with top edges.
In a large bowl, combine the sugar, flour, cinnamon and nutmeg. Add apples and pears; toss to coat. Spoon over crust.
Roll out remaining pastry; make a lattice crust. Trim and seal edges. Brush lattice top with egg white.
Bake at 375° for 40-45 minutes or until golden brown. Cool on a wire rack.