Place the flour, salt, and butter in a food processor and whiz or pulse briefly. Add half the beaten egg and continue to whiz. You might add a little more egg, but not too much as the mixture should be just moist enough to come together.
If making by hand, rub the butter into the flour until it resembles coarse bread crumbs. Then, using your hands, add just enough egg to bring the dough together. With your hands, flatten out the ball of dough, until it is about 3/4-inch thick, and then wrap in plastic wrap, or place in a plastic bag, and leave in the refrigerator for at least 30 minutes or, if you are pushed for time, in the freezer for 10 to 15 minutes.
Sweet shortcrust pastry: In place of the pinch of salt, use 1 tablespoon of icing sugar.
Sour cream shortcrust pastry: Replace the egg with 2 tablespoons of sour cream or creme fraiche, adding just enough to bring the dough together.
Recipe courtesy of Rachel Allen