2) Turn the dough out into a 23-cm tart pan and spread it out evenly with an off-set spatula. Dip the spatula in a little warm water to help smooth and even the surface of the shortbread. Cover and freeze until firm, about 20 mins.
3) Preheat the oven to 160C/Gas 3. Put the tart pan on a baking sheet and prick the dough all over with a fork. Generously sprinkle the top of the shortbread with sugar. Toss the remaining whole almonds in the egg white; evenly arrange and press the nuts into the dough around the edge of the pan. Bake until golden brown and set, about 70 mins.
4) Cool the short bread in the pan on a rack for 10 mins. Carefully remove the tart ring and cut the shortbread with a sharp knife into wedges, each with a nut. Cool the shortbreads on a rack completely. (Alternatively present the shortbread as 1 large cookie. Cool the shortbread in the pan completely and remove the tart ring.)
6) Serve cold. Store in a tightly sealed container for up to 5 days.