Place the flour and butter in a food processor and whiz briefly. Add half the beaten egg and continue to whiz for another few seconds or until the mixture looks as though it may just come together when pressed. You might need to add a little more egg, but not too much as the mixture should be just moist enough to come together. Reserve any leftover egg to use later.
With your hands, flatten out the ball of dough until it is about 2cm thick, then wrap in cling film or a plastic bag and chill for at least 30 minutes.
Preheat the oven to 180°C/350°F/ Gas Mark 4. Roll out the pastry on a lightly floured surface and use to line a 23cm tart tin with foil, greaseproof paper or baking parchment. Fill with baking beans or dried pulses, then place in the oven and bake blind for 15–20 minutes or until the pastry feels dry in the base. Take it out of the oven, remove the baking beans and paper, brush the base of the pastry with any leftover beaten egg then bake for another 3 minutes or until lightly golden. Remove from the oven and set aside.
For the filling, whisk together the eggs and cream. Stir in the salmon and chives and pour into the pastry shell. Place in the oven and cook for about 30-40 minutes until the top is golden and the mixture is set.
As the tart bakes, make the horseradish cream. Mix all the ingredients together in a bowl, season and place in the fridge until you’re ready to serve.
When the tart is baked, remove from the oven and place on a wire rack to cool. Allow to cool, then cut in to slices and serve with a spoon of the horseradish cream.