Melt the butter in a heavy pan, add the salt pork and fry lightly; add the vegetables, bay leaf and thyme and fry to a light brown colour. Sprinkle with the flour and mix in; cook until light brown then add the purée or fresh tomatoes previously squashed. Mix in the stock, add the crushed garlic, salt, sugar and pepper and bring to the boil whilst stirring.
Cover with a lid and place in a moderate oven to cook very gently for 1½–2 hours. When cooked, pass through a fine strainer into a clean pan, stir and reboil for a few minutes; pour into a basin and coat the surface with butter to prevent the formation of a skin.