Step 1 Place the flour, salt, nutmeg and lemon zest in a food processor and pulse to combine. Cut the butter into 1-inch cubes and add the cubes to the flour, pulsing 4 to 6 times to break up the butter.
Step 2 Combine the vinegar and egg yolk in a measuring cup and add enough ice water to bring the volume up to one-half cup. Add the liquid in a steady stream to the food processor, while pulsing, until the flour looks crumbly and damp, 25 to 30 pulses. The crumbs should adhere when you gather them together with your fingers.
Step 3Turn the dough out and divide into two equal pieces. Wrap each in plastic wrap and press into a disk; refrigerate for 30 minutes to 1 hour.
Step 4Roll out one piece into a 12-inch circle, one-eighth-inch thick. Trim the edges flush with the rim of a 9-inch pie pan, place the dough circle into the pan and gently press the bottom and sides to fit. Roll out the other piece to a one-eighth-inch thickness and cut leaf shapes out of it. The leaves can be cut using a leaf-shaped cutter, or by hand using a stencil (ours was 1 inch by 3 inches) and paring knife. Using the back of a dinner knife, press a pattern into each leaf: Press one crease down the center, and 5 or so on each side of the crease. Mix a little water into the reserved egg white and, using a pastry brush, brush a little of the mixture around the edge of the pie crust. Press the leaves around the edge of the crust, overlapping them slightly and using the wash to adhere them, then brush the assembled crust with the wash. Freeze the pie crust for at least several hours and up to overnight.