With your fingers, rub the flour, salt and butter and into fine breadcrumbs. Add the water and draw the mixture together to create a moist pastry. Knead lightly, wrap with cling film and chill for at least 20 minutes.
Preheat the oven to 190'C.
Brush a deep, loose-bottomed, 15 cm tart tin with oil. Cut a small ball of pastry and keep to one side. Roll out the rest on a lightly floured surface into a large circle 20cm in diameter.
Lift the pastry into the tin using the rolling pin and press into the sides. Press any cracks together and let the excess overhang. Use the small ball of pastry to press the tart case into the flutes. Then prick the base lightly with a fork and chill the tart case for at least 30 minutes.
Cut out a circle of parchment paper larger than the tart, scrunch it up, unravel and press into the case.
Fill the case with baking beans, stand on a baking sheet and bake for 20 minutes. Remove the paper and beans, brush the base with egg yolk and return the case to the oven for 10 minutes until crisp and golden. Remove from the oven.
Reduce the oven temperature to 140'C.
In a bowl whisk together the eggs, egg yolks, milk and cream.
Meanwhile, bring a saucepan of salted water to the boil. Slice the leeks, cook until tender and then refresh in ice water. This will stop the cooking process as well as keeping the colour in the leek. Drain any excess water from the leeks and grate the cheddar cheese. Mix the cheese and cooked leeks into the egg mix. Check the seasoning and adjust if necessary.
Pour into the blind baked tart shell and bake for 20-30 minutes or until cooked. The whole of the tart should wobble slightly. If just the centre wobbles it is a sign of an undercooked tart. Allow to cool slightly before serving.
Wash the watercress and remove the tough stalks. Spin in a salad spinner.
Make the dressing by whisking together the salt, mustard and red wine vinegar. Slowly add the olive oil, whisking continuously. Finish with the walnut oil and then taste the vinaigrette. Adjust the seasoning if necessary.
To serve dress the watercress with the vinaigrette and serve alongside the quiche.
«The lower the temperature the tart is cooked the smoother the end result. However bear in mind reducing the temperature on the oven will increase the cooking time.»