Place the hazelnuts in a food processor and blitz to finely chop. Add the flour again and blitz until the nuts are finely ground. Add the caster sugar.
Cut the butter into cubes and add to the processor, blitz until the mixture resembles fine breadcrumbs.
Add the egg yolk, and 2 tbsp of water and blitz until the mixture comes together to form a dough , adding a little more water if required.
Roll out the pastry and use to a line a 23cm/9in tart pan. Prick the base of the pastry case with a fork and chill for 30 minutes.
Preheat the oven to 200℃ (180℃ fan)/400-f/gas mark 6. Line the pastry case with a sheet of baking parchment and fill with baking beans.
Place the tart pan on a baking sheet and bake for 10 minutes. Remove the parchment and beans and return to the oven for 10 minutes.
Meanwhile make the filling
Finely grate the zest from 3 of the oranges. Squeeze the juice from the orange until you have 175ml/6floz (¾ cup).
Beat the eggs and sugar together with a fork or balloon whisk until combined, then whisk in the juice, followed by the cream. Strain the mixture through a sieve, then stir in the orange zest.
Reduce the oven temperature to 180℃ (160℃ fan)/350°F /gas mark 4. Remove the pastry case from the oven and brush with a thin layer of lightly beaten egg white. This will seal the case and prevent the mixture from leaking out.
Carefully pour the orange mixture into the cooked pastry case and return to the oven. Bake for 30 minutes or until the filling is set and just beginning to colour slightly.
Allow to cool and chill until required. Serve plain or if preferred decorate with orange slices and a sprinkling of chopped hazelnuts.
Not suitable for freezing.Nutrition information is approximate and is meant as a guideline only.