In a 4 litre pot of water place the celery, salt, black pepper and chicken breast into cold water. Over low heat bring the stock to a gentle simmer for 10 minutes then turn off heat and allow to cool completely in the water. Once cool, shred chicken meat into a mixing bowl.
Finely chop apricots and onion. Add the remaining ingredients to shredded chicken, mix well and adjust seasoning with lemon juice and salt if needed.
Slice the baguette in half and butter, place chicken mix onto the baguette and top with lettuce. Serve immediately.