To make pastry, sift flour into a large bowl.
Using a pastry blender or 2 knives, cut in butter until evenly distributed and mixture resembles breadcrumbs.
With a fork, lightly mix in sugar and egg to make a dough.
Press into a ball and wrap in foil or plastic wrap.
Refrigerate 30 minutes.
To make filling, wash and halve plums; remove pits.
Cut fruit into quarters.
Preheat oven to 400°F (205°C).
On a floured surface, roll out dough to fit a 10-inch (25-cm) flan tin with a removable bottom.
Place dough in tin without stretching.
Pierce sides and bottom of pastry shell all over with a fork.
Arrange plums in a rosette pattern to fill pastry shell.
Sprinkle with half the sugar crystals.
Bake 30 to 35 minutes; cover with foil if pastry is becoming too brown.
Cool slightly in tin.
Remove from tin and sprinkle with remaining sugar crystals.