Stir about a cup of chicken stock into the flour to make a smooth paste. Bring the remaining stock to a boil, whisk in the flour paste, and continue stirring until the soup thickens. Add the leeks, celery and peas with the sugar, salt and pepper. Cover the soup and simmer until the vegetables are tender, 15 to 20 minutes. Five minutes before the end of cooking, stir in the lettuce. Croutons: Trim the crust from the bread and cut it into 1/4 inch dice. Heat enough butter or oil in a frying pan so that the croutons will float. Fry the croutons over medium heat until they are evenly browned on all sides. Remove them with a slotted spoon and drain them on paper towels. To serve, taste the soup and adjust the seasoning. Spoon it into bowls and serve the croutons separately.