Peel the onion and slice finely. Put it in a large saucepan with the butter and 2tablespoons of water. Cook and soften over a low heat for 10–15 minutes, without browning the onion.
Wash the tomatoes and remove their stalks. Cut each one into four or six pieces and add to the softened onion (1) together with the chopped clove of garlic, thyme and sugar. Increase the heat and let the tomatoes melt and soften for about 10 minutes (stirring with a wooden spoon). While the tomatoes are cooking, bring ¼litre (scant half-pint) of water to the boil and dissolve the bouillon cube in it. Add this liquid to the tomatoes and boil for 2–3 minutes.
Remove the sprig of thyme and pour the contents of the saucepan (2) into the liquidiser jar or through a mouli-légumes placed over a saucepan. Purée or sieve thoroughly. Strain through the wire sieve, return to the saucepan and bring to the boil.
Meanwhile, whisk the 5tablespoons of cream and the egg yolk together in a large bowl. When the tomato mixture has reached the boil, pour it into the bowl and whisk thoroughly.
Return the contents of the bowl to the saucepan and place over a gentle heat, whisking all the time. Remove the pan just before the soup comes to the boil. Whisk for a further 2 minutes away from the heat. Taste, and add the necessary salt and pepper. Strain the soup through a fine sieve into a tureen and serve immediately.