Fill a fish poacher with enough water to cover the whole fish. Add the celery stalks, carrot, onion, bay leaves, peppercorns and vinegar and bring to the boil. Reduce the heat to simmer, cover and cook for 20 - 25 minutes.
Wash the fish and pat dry. Carefully lower the fish into the water. Bring the water back to the boil. Cook gently for 5 minutes then turn off the heat and leave the fish in the water for at least 1 - 1 Ĺ hours.
Lift the fish gently out of the water and remove the skin from the top side. Gently ease it onto a platter dressed with green leaves.
Arrange lemon slices and fresh dill in and around the platter for serving.
Note: retain the water from poaching and set aside to cool before freezing. It will make a delicious base for fish casseroles and soups.