Soak the clams in cold water for about 20 minutes in a few changes of water then give them all a good slosh about until all the grit and sand has been cleaned away.
Make the green sauce by blitzing together the parsley, a good load of garlic oil and a generous splash of sherry vinegar. Season well, taste and perhaps season a little more, not forgetting that vinegar is a seasoning and brings out flavours too, so adjust as you like. You may want it sharper, saltier or sweeter with more oil.
Cook the potatoes in salted, boiling water until tender. About ten minutes. Drain them and run them under cold water to stop them cooking any more.
Heat the garlic oil in a large, heavy saucepan, something like a le Creuset casserole.
Finely chop the onion and gently sauté in the oil, well seasoned with salt, until soft and just starting to think about turning golden.
Add the potatoes and cook them for a few minutes until they start to catch here and there and colour a little.
Stir in the flour and add a splash of sherry vinegar then stir in a good splash of water until you have a thick sauce the consistency of double cream.
Add the clams and cook with the lid on for about five minutes, until they’ve all opened. Or at least until all the ones that are going to open, open. Throw the others away.
Add the green sauce to the pan, stir well, warm through and serve with some more chopped parsley and a little bread to mop up the incredible juices.