In a mixing bowl, toss together the chicken, celery, spring onions and herbs. Set aside.
In a small bowl, whisk together the mayonnaise, lemon juice, mustard, salt and pepper to taste. Add to the chicken and mix gently until combined. Refrigerate until ready to serve.
Build sandwich by placing lettuce, bacon, tomatoes and salad on top of one slice of bread. Top with the second slice of bread. You can secure with a cocktail stick if you prefer. Alternatively, serve on a bed of lettuce with sliced tomatoes, in half an avocado.