Slice the tomatoes and peel the onion very thinly. Melt the butter in a saucepan, add the onion, tomatoes, and potatoes, after mashing them. Pour in the stock or water and meat extract. Boil all gently for fifteen minutes. Rub the soup through a hair sieve, or a fine wire sieve, if time is very short. Rinse out the saucepan, pour back the soup, and re-heat it. Mix the flour thinly and smoothly with a little cold water, strain it into the boiling soup. Re-boil it to cook the flour, add a careful seasoning and the parsley, and pour the soup into a hot tureen.
The soup should be as thick as good cream, so, if too thick, add a little more stock or mixed flour and water, according to whether or not the tomatoes are juicy and the potatoes watery.