Making the pastry:
Preheat oven to 180°C/Gas 4. Place flour and butter in a food processor. Add half a teaspoon of salt and mix until the mixture forms fine crumbs. Pour in water through the feeder tube and pulse again so that the pastry comes together. Knead lightly on a lightly floured surface for a few seconds to make a smooth dough. Wrap in cling film and chill for at least 10 minutes before rolling it out.
Making the filling of the tart:
Roll out the pastry on a lightly floured surface and use pastry to line a loose-bottomed 21 cm (8 ½ inch) fluted flan tin that is about 4 cm (1 ½ inches) deep. Use a rolling pin to lift the pastry into the tin, pressing well into sides and letting the pastry overhang a little as this prevents shrinkage. Chill for another 10 minutes for the pastry to rest.
Prick the pastry base with a fork, then line with a circle of oiled foil or non-stick parchment paper that is first crumpled up to make it easier to handle. Fill with baking beans or dried pulses and bake for 10 minutes until the pastry case looks ‘set’, but not coloured. Carefully remove the foil or paper and lower the temperature to 160°C/325°F/Gas 3, then brush with the egg wash to form a seal.
Return to the oven for another 5 minutes or until the base is firm to the touch and the sides are lightly coloured.
Beat together the cream in a bowl with the milk, chili sauce, eggs and egg yolks until well combined. Stir in the chives and season to taste. Scatter the smoked salmon, red onion and sliced leeks in the bottom of the pastry case, then pour in the cream mixture and bake for about 25 minutes until the filling is just set but still slightly wobbly in the middle.
Leave to rest for 5 minutes in the tin, then remove and trim down the excess pastry.
Making the lemon cream sauce:
Place the cream in a small pan and bring to the boil, then reduce the heat and allow to simmer for 12 minutes, until well reduced and thickened. Remove from the heat and whisk in the mustard, the lemon rind and juice and season to taste. Stir in the chives and keep warm over a low heat.
To serve, carefully cut the tart into slices. Arrange on plates with the lemon cream sauce.