Preheat the oven to 200°C.
Roll out the pastry on a lightly floured surface to 1cm thick and cut rounds of pastry slightly larger than individual fluted flan tins. Line the flan cases using the pastry, pressing in well and prick the bases. Trim any overhanging pastry and chill for 10 minutes.
Meanwhile, melt the butter in a large, heavy-based saucepan over a medium heat and sweat the leek for 6-7 minutes, stirring occasionally. Remove from the heat, stir in the chopped thyme and set to one side. Line the pastry cases with nonstick baking paper and fill with baking beans. Bake in the oven on a baking sheet for 10-12 minutes until golden.
Remove the paper and beans at this point, brush the bases with the beaten egg yolk and return to the oven to allow the base to cook and brown. Meanwhile, make the filling by whisking the Crème fraîche and the cream together. Beat in the eggs one at a time and season well with salt and pepper.
Remove the pastry cases when ready and reduce the oven to 180°C at this point. Arrange the smoked salmon and the leek mixture evenly in the base of each pastry case. Pour the filling into each one so that it comes three quarters of the way up.
Bake for 20 minutes until the filling has set and the tops are golden-brown in colour. Remove and serve immediately with a sprig of thyme on the side.
*This dish is very induglent, so shouldn't be eaten too often. But it's perfect for those special occasions!