Thaw the fish overnight in a colander in the refrigerator. Scale and score with a sharp knife.
Place the vegetables in a saucepan and the fish on top. Add the water and olive oil and simmer over a medium heat for 45 minutes.
Transfer the fish to a serving dish and carefully remove the bones.
Remove the celery and the onion and purée the stock and the vegetables with a hand blender until smooth, adding the lemon juice. Pass through a sieve.
Serve in a dish with pieces of the fish and freshly ground pepper.