Sift the flour and the salt into the bowl of a food processor.

Add the butter and blitz for a few seconds until the mixture resembles wet sand.

Add half of the beaten egg, and blitz for a few more seconds.

Add more egg as necessary, until the mixture comes together into a ball.

Wrap the dough in some plastic wrap and chill in the fridge for about 30 minutes.

Lightly grease a 23 cm (9 inch) tart tin or pie dish (with a loose bottom).

Preheat the oven to 180°C (350°F) (without fan).

Roll out the pastry between two sheets of baking paper.

Line the pan with the pastry and trim the edges.

Place the prepared pie dish in the fridge for about 15 minutes to chill.

Line the prepared pie dish with some baking paper, fill with baking beans or pie weights, and blind bake for about 25 minutes, or until the pastry feels slightly dry to touch.

Remove the baking beans or pie weights, and let the pan cool on a wire rack.

Melt the butter in a small pan over medium heat.

Add the onion and sauté for about 10 minutes until it is soft and lightly golden.

Line the base of the pastry-lined pie dish with the cooked onions.

Thickly slice the potatoes and arrange them over the onions.

Sprinkle with some fresh dill, and season with salt and pepper.

Arrange the slices of smoked salmon on top of the potatoes.

In a large jug, whisk together the eggs and cream, and season with salt and pepper.

Pour this mixture into the pie dish.

Bake the tart in the an oven at 180°C (350°F) (without dan) for about 30-40 minutes, or until it is just set in the centre.

Serve warm or cold.