Place shallots, wine, 1 tablespoon (15ml) lemon juice, 1⁄2 cup (125 ml) water, peppercorns and salt in a pan; bring to a boil. Add chicken fillets and return to a boil. Reduce heat to low, cover pan and cook chicken 15 minutes, or until cooked through. Remove pan from heat. Leave chicken to cool in cooking liquid.
Halve avocados and remove seeds. Remove 4 teaspoons of flesh from each avocado half. Dice, place in a bowl. Mix with 1 tablespoon (15ml) lemon juice to prevent browning. Drizzle cut surface of avocado halves with 1 tablespoon (15ml) lemon juice.
Dice celery. Peel cucumber, cut in half lengthwise and remove seeds with a small spoon. Dice cucumber. Drain chicken and cut into small cubes. Add chicken, celery and cucumber to diced avocado and stir gently to combine.
To make dressing, whisk sour cream, mustard, oil, vinegar, sugar and remaining 1 tablespoon (15ml) lemon juice in a small bowl. Add salt and pepper to taste. Stir dressing into chicken and vegetables. Spoon mixture into avocado halves.
Dust avocados with a little cayenne pepper. Cut sorrel leaves into thin strips, if using, and scatter over avocados.