The British traditionally serve freshly prepared hot English mustard with mild-flavored cold meats, such as ham or boiled tongue. We've paired this mustard's piquancy with poached chicken.
Place carrots, celery, onions, parsley and peppercorns in a stockpot. Rinse chicken with cold water, and pat dry. Add chicken. Fill stockpot with enough cold water to cover chicken by 1 inch.
Set over medium-high heat, and bring water to a boil. Reduce heat, and allow to simmer, skimming off any foam that rises to the surface, for 1 hour.
Remove chicken from stock; set aside. Strain stock through a very fine sieve or chinois. Discard solids. Transfer stock to a metal bowl. Set bowl in another bowl, full of ice water. Let stand until stock is completely chilled, replacing ice if necessary, and fat can be skimmed off surface of stock.
Meanwhile, when chicken is cool enough to handle, remove skin from chicken and discard. Carefully pull meat off chicken bones; it should fall away quite easily, keeping the meat in large chunks when possible.
Return 5 cups stock to stockpot or large saucepan. Bring to a boil; adjust seasoning with salt. Reduce heat to a simmer; simmer until stock has become very flavorful and reduced by 1 cup, about 15 minutes. Add pieces of chicken; cook just until chicken is heated through, about 5 minutes.
Toast bread until golden, and brush with olive oil. Serve chicken in a wide bowl with a ladle of hot broth. Add a handful of watercress to each bowl. Place a spoonful of mustard on toast, sprinkle with salt, and serve on the side.
This recipe yields excess chicken stock. The excess can be kept in plastic containers and frozen for up to several months.