Grind the walnuts and mix with the flour. In another bowl, mix the butter, sugar and egg yolk. Amalgamate together all the mixtures, minding not to knead the dough too much. Leave to rest in a cool place for two hours or so. In the meanwhile, caramelise the sugar with the lemon juice. In another pan, boil the cream with the honey and coffee, then blend it in the sugar caramel. Add the remaining ground walnuts and allow to cool. Line the bottom and sides of a low-sided mould with the dough. Fill with the walnut and caramel mixture and cover with a layer of dough. Bake at 350° F for 30 minutes or so.