Heat the oven to 170 C / 340 F.

Melt the butter in a medium frying pan over medium-low heat. Add the sliced leeks and cook for 8-10 minutes, stirring occasionally, until completely soft and just starting to turn golden at the edges.

Remove from heat, press the leeks lightly with a spoon to release any liquid, then discard the liquid. Set leeks aside to cool for 5 minutes.

In a mixing bowl, whisk together the eggs, egg yolk, double cream, and milk until smooth and uniformly combined.

Add the lemon zest, chopped dill, and black pepper. Taste and add a very small pinch of salt if needed - the salmon adds significant saltiness.

Place the blind-baked pastry shell on a baking sheet. Spread the softened leeks evenly across the base.

Pour the custard mixture over the leeks slowly, filling to about 5 mm below the pastry rim.

Lay the smoked salmon slices across the top of the custard, gently pressing them just below the surface so they don't curl during baking.

Bake at 170 C / 340 F for 22-25 minutes, until the edges are set and the center still has a slight wobble when the tin is nudged.

Remove from the oven and rest on a wire rack for 10 minutes before slicing. The custard will firm up fully as it cools.