1. To make pastry, place flour, icing sugar and salt in a bowl. Rub in butter until mixture resembles coarse crumbs. Add egg and enough cold water to mix into a dough. Wrap the pastry in plastic clingfilm and refrigerate for 30 minutes.
2. Roll dough out to a 3mm thickness and line 6 x 12cm tart pans with pastry. Return to the refrigerator.
3. Place blueberries and rhubarb together in a saucepan over a gentle heat. Mix together the cornflour and sugar and sprinkle over the fruit. Allow fruit to boil and the liquid to thicken, stirring gently. Remove from heat and set aside to cool. This can be done the day before if you like.
4. Cut strips of pastry about 1cm wide and 13cm long.
5. Spoon fruit into pastry cases and make a lattice pattern on top with pastry strips. Brush with egg yolk and bake for 15 to 20 minutes until pastry is golden and cooked.

Cook's tip: If your fruit mixture is too thin, cook it for longer and allow it to reduce, or thicken it with a little more cornflour mixed with cold water. Serves 6, ready in 50 mins, plus time to refrigerate