Tear into bite-size pieces and arrange on a serving platter. Divide chicken into pieces, leaving skin on where possible and cutting larger sections so that the pieces are roughly uniform. Arrange half the chicken pieces on the lettuce, top with half the avocado. Repeat with remaining chicken and avocado.
Scatter celery slices over.
Place mayonnaise, yoghurt, vinegar, salt and pepper in a small screw-top jar and shake well. Drizzle over salad. Garnish with pecans. Serve immediately with warm wholemeal bread rolls or slices.