How to Make It
In a large glass or stainless-steel bowl, whisk together the lime juice, mayonnaise, mustard, salt, and black pepper. Add the oil slowly, whisking.
In a medium glass or stainless-steel bowl, toss the chicken, cucumber, bell pepper, onion, mango, and tarragon with half the dressing. Toss the greens into the remaining dressing. Put the greens on plates and top with the chicken salad.
The skin of a ripe mango is yellow and red, and its flesh yields to gentle pressure. If you purchase a mango that is underripe (green and firm), put it in a paper bag and keep it at room temperature to ripen. Once the mango is ripe, you can refrigerate it for a couple of days before using.