Scoop the lardons out of the pan onto some kitchen paper and tip away most of the fat. Add the onion and celery to the pan with the residual fat, and fry them slowly and gently until they are very soft but not brown (there will be some brown bits left by the lardons – don’t worry about these, they add flavour). Add the lardons, potato, bay leaf and flour, and stir until the flour is thoroughly mixed with any fat. Add the milk a little at a time, stirring to make sure the flour doesn’t turn into lumps. Add the liquid from the clams and bring to a simmer, then cook until the potato is tender.