1) Preheat the oven to 180C/Gas mark 4. Place the chicken breasts, skin side up, on a baking sheet and rub them with olive oil. Sprinkle generously with salt and pepper. Roast for 35 to 40 minutes, until the chicken is cooked through. Set aside to cool. 2) When the chicken is cool, remove and discard the skin and bones and cut the chicken into 2cm dice. Place the chicken in a bowl and add the mayonnaise, tarragon, celery, 2 tsp of salt, and 1 tsp pepper. Toss well. 3) To assemble, spread a little mayonnaise on half the bread slices, top with the chicken and green salad mix, and cover with the remaining slices of bread. Cut in half and serve.