1. In a 1 1/2- to 2-quart pan, mix granulated sugar and flour. Whisk in milk and lemon peel. Stir over medium-high heat until mixture boils and is thickened and smooth, 4 to 7 minutes. Remove from heat. In a small bowl, beat the 2 egg yolks lightly to blend. Whisk about 1/4 cup of the hot milk mixture into the yolks, then whisk egg mixture into milk mixture in pan. Nest pan in a bowl of ice water and stir occasionally until cool, about 10 minutes.
2. Coat a 9-inch pie pan, including rim, with cooking oil spray or butter. Divide butter pastry into two portions, one 2/3 of the total and the other 1/3. Press the larger portion into a flat disk and place between two sheets of plastic wrap (about 15 by 15 in.; overlap sheets, if necessary, to make wide enough). Roll into a 13-inch round about 1/8 inch thick. Peel plastic wrap off one side of dough; invert and center pastry over prepared pie pan and gently ease into pan. Peel off remaining plastic wrap. Trim pastry edges flush with rim. If there are any tears in pastry, press dough together to repair. Spread the cooled custard evenly in pastry.
3. Gather pastry scraps and add to remaining portion. Press into a disk and place between two sheets of plastic wrap (about 12 by 15 in.). Roll into a 10-inch round about 1/8 inch thick. Peel plastic wrap off one side of pastry, invert, and center over pie pan. Peel off plastic wrap. Trim off excess dough flush with rim. With the tines of a fork, lightly press pastry edges together on rim.
4. Beat egg white lightly to blend and brush pastry just to coat. Sprinkle evenly with almonds and pine nuts; press nuts lightly into surface.
5. Bake in a 350º regular or convection oven until golden brown, 30 to 40 minutes. Cool on a rack at least 1 1/2 hours before serving. When completely cool, cover and chill. Serve cool or cold. Shortly before serving, sift powdered sugar over the top.
Butter pastry: In a food processor or a bowl, whirl or mix 2 cups all-purpose flour, 6 tablespoons sugar, and 1 1/2 teaspoons baking powder to blend. Add 1/2 cup (4 oz.) cold butter, cut into 1/2-inch chunks. Whirl or, with a pastry blender, cut butter into flour mixture until cornmeal-size pieces form. Add 1 large egg and 1 large egg yolk; whirl or mix with your hands until dough comes together. Shape into a ball.