For the pastry, put the flour, butter, egg yolk and 4 tsp cold water into a food processor. Using the pulse button, process until the mix binds. Tip the pastry onto a lightly floured surface, gather into a smooth ball, then roll out as thinly as you can. Line a 23 x 2.5cm loose-bottomed, fluted flan tin, easing the pastry into the base. Trim the pastry edges with scissors (save any trimmings) so it sits slightly above the tin (if it shrinks, it shouldn’t now go below the level of the tin). Press the pastry into the flutes, lightly prick the base with a fork, then chill for 10 mins. Put a baking sheet in the oven and heat oven to 200C/fan 180C/gas 6.
Line pastry case with foil, shiny side down, fill with dry beans and bake on the hot sheet for 15 mins. Remove foil and beans and bake for 4-5 mins more until the pastry is pale golden. If you notice any small holes or cracks, patch up with pastry trimmings. You can make up to this point a day ahead.
While the pastry cooks, prepare the filling. Heat a small frying pan, tip in the lardons and fry for a couple of mins. Drain off any liquid that comes out, then continue cooking until the lardons just start to colour, but aren’t crisp. Remove and drain on paper towels. Cut three quarters of the cheese into small dice and finely grate the rest. Scatter the diced cheese and fried lardons over the bottom of the pastry case.
Using a spoon, beat the crème fraîche to slacken it then slowly beat in the double cream. Mix in the beaten eggs. Season (you shouldn’t need much salt) and add nutmeg. Pour three-quarters of the filling into the pastry case.
Half-pull the oven shelf out and put the flan tin on the baking sheet. Quickly pour the rest of the filling into the pastry case – you get it right to the top this way. Scatter the grated cheese over the top, then carefully push the shelf back into the oven. Lower the oven to 190C/fan 170C/gas 5. Bake for about 25 mins, or until golden and softly set (the centre should not feel too firm). Let the quiche settle for 4-5 mins, then remove from the tin. Serve freshly baked, although it’s also good cold.