Brush the Dijon mustard onto the chicken breasts and season with salt and freshly ground black pepper. Place the chopped walnuts into a large bowl. Add the chicken, mustard side down and coat in the chopped walnuts. Place the chicken onto a baking tray and drizzle with a tablespoon of olive oil. Bake in the oven for 35-40 minutes, turning around half way through cooking time. Meanwhile, place the sliced celery, walnut halves, green apples and grapes into a large bowl and season well with salt and pepper. Place the olive oil, lemon juice and zest, tarragon and mayonnaise into a small bowl and mix together well. Pour the mayonnaise dressing over the fruit and walnuts and mix together well. When the chicken is cooked through, remove from the oven a slice thickly. Mix into the salad whilst still warm and serve immediately.