Blanch and refresh 10 nice lamb’s sweetbreads.
On the bottom of a thick well buttered shallow pan, place 1 kg (2 lb 4 oz) sliced and blanched Spanish onions; season with salt and pepper; arrange the sweetbreads on top and add a very little white stock. Cover with slices of salt pork fat and a lid and cook gently in the oven.
When ready, transfer the sweetbreads to a clean pan along with 10 small cooked mushrooms and 2 medium-sized truffles cut in slices.
Pass the onions through a fine sieve and mix the resultant puree together with a few tablespoons of well creamed Sauce Bechamel. Pour this over the sweetbreads and the garnish; reheat without boiling and add a little butter away from the heat
Pour the whole into a large Vol-au-vent case and place on a serviette on a dish to serve.