Process flour, butter and a pinch of salt in a food processor until the mixture looks like breadcrumbs.
Whisk the egg and 1 T chilled water in a bowl until combined. Add to the flour mixture while the food processor is running. Process until the dough forms large clumps before it form s a ball.
Turn the dough out onto a work surface eg onto baking paper, and knead gently to bring together. form into a disc for a round tart or a log for a rectangular tart. Wrap in plastic wrap and refrigerate overnight or for at least 2 hours.