Cook the chicken with the carrot, celery bulb, leek and salt until you can pull the leg off the chicken easily, but not too soft either. Take the chicken out of the broth, let it cool, remove the skin and bones and cut into bite-sized pieces.
Heat the butter in a saucepan, add the flour and sweat while stirring. Slowly add a liter of chicken stock while stirring, bring to the boil and simmer for 10 minutes. Stir frequently so that nothing begins to stick. If the sauce is too thick, add a little more stock.
Season to taste with lemon juice, pepper, nutmeg and sugar.