Place chicken breast in a saucepan, cover with water, season to taste with sea salt, then bring to the boil over medium heat. Cook for 8 minutes, then turn off heat and leave for 10 minutes or until chicken is just cooked. Remove chicken and cool.
For mayonnaise, process egg yolk, mustard and lemon juice in a food processor for 1 minute or until pale and frothy then, with motor running, add olive oil in a steady stream until mixture is thick and emulsified. Season to taste. Makes about 1 ¼ cups.
Using fingers, finely shred chicken meat and combine in a large bowl with mayonnaise, celery, capers, garlic and herbs and season to taste. Spread chicken mixture over half the bread slices, top with remaining slices, cut each into quarters and serve immediately.