Scald the tomatoes in boiling water for a few seconds, then plunge into cold water to loosen the skins. Peel away the skins, halve the tomatoes and scoop out the seeds. Chop the tomato flesh roughly. Scrape and chop the carrot and peel and chop the onion.
Sauté the onion and carrot in the melted butter for 5–6 minutes. Add the chopped tomatoes and sauté for a further 5–6 minutes to draw the juices. Stir in the sugar, a seasoning of salt and pepper, the stock and the bouquet garni. Cover the pan with a lid and simmer for 30–40 minutes or until the vegetables are soft.
Remove the bouquet garni and press the sauce and vegetables through a sieve. Stir in the cornflour blended with the water. Return to the heat and stir until boiling. Check seasoning before using.