To make pastry, sift flour into a large bowl.
Using a pastry blender or 2 knives, cut in butter evenly.
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.
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.
Bake blind,20 to 25 minutes or until golden.
Cool slightly in tin then transfer to a rack to cool completely.
To make filling and decoration, wash and pit plums.
Simmer with sugar and 3 tablespoons water in a medium saucepan over low heat until soft.
Dissolve gelatin in 3 tablespoons water over low heat.
Mix cornstarch with a little cold water.
Add to plums.
Bring plum mixture to a boil, stirring constantly.
Stir in walnuts.
Mix dissolved gelatin with plum mixture and pour into pre baked pastry shell.
Refrigerate until set.
Whip cream until stiff; put into a pastry bag fitted with a fluted nozzle.
Mark flan into 12 portions.
Decorate with piped whipped cream and walnut halves.