Bring poaching water and 1/4 teaspoon salt to a gentle simmer in a saucepan. Add chicken, partly cover and cook for 15 to 20 minutes until the thickest centers reach 165°F.
Drain chicken, cut into small pieces and spread in a shallow container. Refrigerate promptly for about 60 minutes until 40°F or colder; this chilling time is included in preparation.
Toast hazelnuts in a dry skillet over medium-low heat for 3 to 4 minutes, stirring, then transfer to a plate and cool completely.
Mix mayonnaise, yogurt, lemon juice, tarragon, pepper and the remaining 1/4 teaspoon salt. Fold in cold chicken, diced apple and cooled hazelnuts.
Put one dry lettuce leaf on each croissant bottom and divide the salad over the leaves. Replace the tops without pressing.
Serve cold or refrigerate at 40°F or below until serving, transporting with ice packs if taking the sandwiches out.