Place the skate in a pan large enough to hold the fish, cover the skate completely in water. Add onion, parsley sprigs, salt, 2 tablespoons of white wine vinegar and some black
peppercorns. Bring to the boil, with the pan uncovered. Reduce the heat and allow to simmer on a low heat for 12 - 15minutes.
Place the skate into an ovenproof serving dish and keep it hot over a low flame or just put it on to the serving plate whilst you make the black butter.
To make the black butter, place the butter into a small frying pan and heat until it foams and begins to turn brown. Remove from the heat immediately, add the lemon juice, parsley and capers and pour instantly over the fish.