Remove the butter from the fridge 30 mins before making the pastry.
Sift the flour and salt onto a work surface. Cut the butter into smallish pieces, add to the flour and lightly rub together to partly combine.
Make a well in the centre, add the water and, using a pastry scraper, work the paste to a rough heap of buttery lumps of dough.
Then work together as briefly as possible into a smooth dough, roll out on a lightly floured surface and use to line a loose-bottomed flan tin that is 2.5cms deep and 24cm across the base.
Prick the base here and there with a fork and chill for 20 mins.
Preheat the oven to 200 degrees or gas mark 6. Line the pastry case with a sheet of crumpled greaseproof paper and a thin layer of baking beans and bake for 20 mins.
Remove the paper and beans and return to the oven for 10 mins. Remove and lower the oven temperature to 190 degrees or gas mark 5.
Put the butter, honey, crème fraîche and rum together in a saucepan and warm gently over a low heat, but don't let it get hot.
Mix the walnuts and sugar together in a bowl.
Lightly whisk the egg yolks, and whisk them into the honey mixture, then stir this into the walnuts and sugar.
Pour the mixture into the pastry case and bake for 40 mins until the tarte is well caramelised.
Leave to cool, and cut into thin wedges and serve with some crème fraîche.