In an ovenproof casserole or Dutch oven melt one tablespoon of the butter and cook the onions until softened, about 5 minutes. Sitr in the remaining butter and the tomatoes. Cook over low heat until very thick, about 20 minutes. Stir in the garlic and 4 cups of the water. Simmer for 25 minutes. Strain. Add enough of the remaining water to obtain the desired consistency. In a small bowl whisk together the cornstarch, sugar and a couple tablespoons of water. Stir into the soup. Season with salt and pepper. Bring to a boil. 15 minutes before serving whisk together the egg yolk and parsley. Stir the cream into the hot soup. Stir a ladleful of soup into the egg yolk mixture. Serve with croutons.
