Melt butter in a heavy sauce pan. Cook onions and celery until tender. Add flour, mustard, nutmeg and pepper and stir, cooking for 2 – 3 minutes. Add chicken stock and simmer for about 20 minutes, stirring occasionally. You may want to use a hand blender to puree the mixture when done. Return soup to stove, adding cream and water and bring close to boiling. Immediately stir in cheese and allow to melt. Add dashes of salt, pepper and Worcestershire sauce to taste.