Preheat the oven to 160C/320F/Gas 2. Put the flour, caster sugar, butter and semolina into a food processor and process until the mixture is thoroughly combined and comes together to form a dough. This can also be done by hand, rubbing the butter into the flour first, then adding the sugar and semolina and working the ingredients together to form a ball.
Roll the dough out on a floured surface to 1-2cm/½in thick. Using a circular cookie cutter or the top of a drinking glass, cut the dough into circles. Place the circles into a lightly greased tin and sprinkle the top with the demerara sugar.
Bake for 30-40 minutes, until the shortbread is pale golden and cooked through. Keep an eye on it in case it gets too brown. Allow the shortbread to cool for a few minutes then carefully lift each shortbread with a small palette knife and place on a wire rack to cool completely. (These can now be stored in an airtight container.)
Meanwhile, make the filling. Place the strawberries, port and sugar into a pan and heat gently until the sugar dissolves, stirring and mashing the strawberries into a chunky salsa. Allow to cool.
Fold together the whipped double cream, yoghurt and whipped egg white.
Fold in the strawberry salsa, leaving swirls.
Place one shortbread circle on a small plate and place on a large dollop of the strawberry cream.
Place one whole strawberry on top of each dollop and serve.