Preheat the oven to 220ºC/425ºF/ Gas Mark 7. Roll out the pastry on a floured surface to the thickness of a £1 coin and line a 4cm deep, 20cm round loose bottom flan tin. Line the pastry with a circle of greaseproof paper and baking beans. Chill for 10 minutes. Bake in the oven for 15 minutes, then remove the paper and baking beans.
Brush the inside of the pastry with egg wash and turn the oven down to 180ºC/350ºF/Gas Mark 4. Mix the eggs and cream together and season. Sprinkle the cheese, tarragon, spring onion and crab over the tart case and pour over the egg mixture. Bake for 25 minutes until the custard is set and the pastry is golden brown. When the tart is cool slice and serve with a simple seasonal salad.