Melt fat in a large heavy saucepan, add diced salt pork, carrots and onion, and cook until they start to turn golden, shaking the pan so that they cook evenly. Sprinkle flour over this mixture and cook gently on a very low heat, stirring frequently, until it takes on a good golden brown colour. Add one-third of the boiling stock together with bouquet garni and garlic, and cook, stirring frequently, until the sauce thickens.
Add half of the remaining stock and cook very slowly over a very low heat, stirring occasionally, for about 1½ to 2 hours, or until the mixture is reduced to approximately half the original quantity. As it cooks, skim off scum and fat rising to surface. Add Tomato Sauce (or concentrate), and cook for a few minutes longer. Then strain through a fine sieve into a bowl, pressing the vegetables against the sieve to extract all their juice.
Clean the saucepan, return the mixture to it, add remaining stock, and continue cooking slowly until the sauce is reduced to about 1 quart, skimming the surface from time to time as needed.
Strain again. Cool, stirring occasionally. Store Espagnole Sauce in a covered jar in the refrigerator until ready to use. A little butter melted over the top will seal it and help it to last even longer.