Preheat your oven to 425 degrees. Open your pie crusts. Use one to line the bottom of a 10-inch tart pan or spring form pan with removable bottom. Turn the top edge of the pie crust over to form an edge so the rim of crust is about three quarters of an inch high. You don't have to be too precise but, if it's too high, your pie crust will droop as it bakes.
In a bowl, mix your rhubarb cubes, the 3/4 cup sugar and the cornstarch with the grated peel. Stir them until they are well combined. You want to make sure the cornstarch is well distributed. Pour the mixture into the crust in your pan.
Now sprinkle the nutmeg across the top of the rhubarb mixture. I like to sprinkle the nutmeg on top instead of mix it in because I want the nutmeg flavor to jump out when I bite into the tart. If you want it to be more subtle you can mix it in. That's also why I didn't specify an exact amount of nutmeg. I love nutmeg and I want it to be a bold flavor in my tart. You can vary the amount to your mood and taste.
Now unroll your other crust. You want to make sure the crust is slightly larger than your tart pan. If it's not, just take your rolling pin and roll it a tiny bit to stretch it so it's slightly larger. You're going to use a pizza cutter to cut strips of dough so your tart can have a lattice top and you'll be able to see your rhubarb peeking through. Cut one-inch strips from your pie crust. Use up the whole crust. Now you're going to begin making your lattice. This is a bit difficult to explain without a picture. Start with your middle pieces--the longest ones. Pick up the longest piece of crust and lay it across the middle of your tart. There will probably be an inch or two laying off the end on each side. Just tuck those along the side of your tart pan, down the sides. Don't worry if it's not perfect. Take the next longest piece and lay it the opposite way, again in the middle. If the first one was north to south, this one is going east to west. Keep going back and forth across your pie. Leave about a half inch between the rows so you can see your rhubarb in between. That way the pieces will get a basket-weave look. When you get to the ends you'll have the pieces that have the rounded edges. You'll use those to cover up along the sides where you've been tucking your edges in. Any time your pieces along the sides seem like they are too long, you can just cut them off and toss away the extra dough. It's nice to have some dough tucked along those edges though. It gives your tart some strength on the sides.
Now you're going to glaze the tart. Put your milk in a cup. Use a pastry brush to stroke the milk over the crust. Then sprinkle the sugar on lightly. You're ready to bake.
Put your tart in the oven at 425 degrees for 12 minutes. Then turn the oven down to 325 and bake it for 20 to 25 minutes more until it's golden brown. If the crust starts to get too brown, just lay a piece of aluminum foil over the whole tart to shield it from getting any darker.
Remove it from the oven when it's brown and let it cool. When you want to remove it from the pan, take a sharp knife and cut around the edge of the crust to loosen it. Then pop off the ring and display your dessert on a nice platter.