Put the potatoes in a steamer, salt them, and set the water boiling. When evenly cooked through, drain and leave until you are just able to handle them, then peel. (Peeling potatoes while still hot is easier and they will accept and absorb the dressing more easily.) Put back into the steamer and leave to keep warm in its residual heat.
To make the dressing, put the mustard, vinegar, seasoning, and a couple of tablespoons of the water in a blender or food processor. Process until smooth and then start adding the oil in a thin stream.
When the consistency has become creamy have a taste. If you think it is too thick, add a little more water; the look should be that of thin mayonnaise. Put to one side.
Poach the skate: Put approx. 1 quart water in a large pan and add to the onion, celery, bay leaf, white wine vinegar, salt and peppercorns. Boil together for 20 minutes. Slip in the skate wing and bring to a gentle simmer. Poach for 10 minutes. Lift out with a spatula, drain for a few seconds, and put on a large plate. Using a small knife, deftly remove both the black and the white skin (including any "thorns" that protrude from just beneat the skin) and remove the flesh with a knife, easing it carefully off either side of its flimsy cartilage, in strands. Transfer to a hot plate and loosely cover with foil – or something or other.
Thinly slice the potatoes into a warmed, shallow serving dish, sprinkle over the spring onions, and spoon over just enough dressing to coat them. Stir together.
Distribute the skate strands over the potatoes, lubricate with a little more dressing, and sprinkle with much parsley.