Preheat the oven to 325° F.
Mix flour and salt in a large bowl. Cut in butter with your fingers until the flour is very mealy and you can't see individual pieces of butter. Do this quickly--you want to keep things cold so you don't end up making paste.
Put 2-3 ice cubes in your water, then drizzle in about 1 ounce of water at a time.
In between drizzles, toss the flour with the water to evenly distribute it. When you get about 3½ ounces of water incorporated, test it by squeezing a bit of the dough together (it won't look like dough until you squeeze it). If it's not staying together, add the rest of the water. If it is staying together, resist the urge to add more water and compress the dough into a disc. Wrap in plastic wrap and refrigerate for 30 minutes.
Roll between two sheets of parchment to about ⅛ - ¼ inch thick.
Cut to fit over your soup crock, and press the edges down around the crock with hot soup in it. Brush with 1 egg beaten with 2 teaspoons of water.
Bake at 325°F for about 15 minutes, or until the crust is golden brown and delicious.
For an added treat, sprinkle a layer of cheese on top of the soup before you put on the crust, or grate on some cheese during the last 5 minutes of baking.
...or, if that seems way too much like work, and I wouldn't much blame you if it does, use frozen ready-made puff pastry, available in the freezer aisle. Thaw it in the fridge overnight, cut to fit and bake according to the package directions."