Rinse the celery hearts and remove and reserve any leaves for use in the sauce.
In an ovenproof dish, arrange the celery hearts in the base, season with salt and pepper and pour over the hot stock. Cover with foil.
Bake in the preheated oven for up to 1 hour until tender.
Remove from the oven and drain the stock, keep the celery hearts warm. Pour half of the stock into a saucepan and reduce by boiling until only a few tablespoons of stock remain. Turn down the heat, pour in the cream and stir continuously until the sauce reduces and thickens. Finely chop the reserved celery leaves and stir into the sauce.
Arrange the celery hearts in a warm serving dish, pour over the sauce and serve immediately.