Place 8litres (1¾gal or 2¼U.S. gal) of the stock in a heavy pan and bring to the boil; add the Roux, previously softened in the oven. Mix well with a wooden spoon or whisk and bring to the boil mixing continuously. Draw the pan to the side of the stove and allow to simmer slowly and evenly.
Meanwhile, place the salt pork in a pan and fry to extract the fat, add the vegetables and flavourings and fry until light brown in colour. Carefully drain off the fat and put the ingredients into the sauce; deglaze the pan with the wine, reduce it by half and also add to the sauce. Allow to simmer gently for 1 hour skimming frequently.
Pass the sauce through a conical strainer into another pan, pressing lightly. Add another 2litres (3½pt or 9U.S. cups) stock, bring to the boil and allow to simmer gently for a further 2 hours. Pass the sauce through a fine strainer and stir occasionally until completely cold.
The next day, add the remainder of the stock and the tomato purée; bring the sauce to the boil stirring continuously with a wooden spatula or whisk, then allow to simmer gently and evenly for 1 hour skimming carefully.
Pass through a fine strainer or tammy cloth and stir occasionally until the sauce is completely cold.