Prepare the Pastry:

Sift the flour and salt into a food processor bowl.

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

Add half of the beaten egg and blitz again until the mixture begins to form a dough.

Add the remaining egg as needed until the dough comes together into a ball.

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

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

Line the Pie Dish:
Lightly grease a 23 cm (9-inch) tart tin or pie dish with a loose bottom.
Roll out the chilled dough between two sheets of baking paper.
Line the prepared pie dish with the pastry, trim the edges, and place it in the fridge for about 15 minutes to chill.

Blind Bake the Pastry:
Line the pastry with baking paper and fill with baking beans or pie weights.
Bake for about 25 minutes, or until the pastry feels slightly dry to touch.
Remove the beans or weights and let the pastry cool on a wire rack.

Prepare the Filling:
In a small pan, melt the butter over medium heat.
Add the onion and sauté for about 10 minutes, until soft and lightly golden.

Assemble the Pie:

Line the base of the baked pastry shell with the cooked onions.

Slice the boiled potatoes thickly and arrange them over the onions.

Sprinkle with fresh dill and season with salt and pepper.

Arrange the smoked salmon slices on top of the potatoes.

Make the Custard Filling:
In a large jug, whisk together the eggs and cream. Season with salt and pepper.
Pour the egg mixture over the layered ingredients in the pie dish.

Bake the Pie:
Bake the pie in the preheated oven for about 30-40 minutes, or until the center is just set and golden.

Serve:
Let the pie cool slightly before serving. It can be enjoyed warm or cold.