Lightly grease a 23 x 15 cm/9 x 6 in shallow tin and make up the shortbread dough according to the recipe instructions.
Press the shortbread dough into the prepared tin and smooth the surface with the back of a warm metal spoon.
Sift the flour into a bowl.
Rub in the butter or margarine until the mixture resembles fine breadcrumbs, then stir in the sugar.
Spread the shortbread base evenly with jam, then sprinkle the crumble over the top.
Bake the biscuit in a moderate oven (170C, 325 F, gas 3) for 1 hour, or until it is lightly browned.
Allow to cool completely in the tin before marking and cutting into fingers.
Variation Mincemeat streusel: substitute 225 g/8 oz mincemeat for the raspberry jam.