Because celery has a very pungent taste it benefits from par-boiling beforehand.
Wash and trim the celery hearts to about 6 in. (15 cm) in length. Halve lengthways and par-boil in salted water for 7 minutes. Drain well. Put the chopped onions and carrots into the bottom of a heavy-based pan or flameproof dish. Lay the celery hearts on top and cover with a good veal stock. Bring to the boil and then braise, uncovered, at gas mark 3 or 4, 160 to 180°C (325 to 350°F), for 1 hour until the celery is very tender. By this time the stock should be of a syrupy consistency. If it is not, remove the vegetables on to a serving dish and keep warm. Boil the stock rapidly until it reaches the right consistency and pour over the vegetables.