Make the dressing first. Mince the shallots and add them to the rest of the all ingredients. Stir well to combine. Or better yet, shake in a jar.
Thinly slice the mango and red onion. Slice up the avocado into wedges and cut in half.
Combine the salad dressing (you might not want to use all of it...use your judgement as to how much. You don't want the salad to wilt under the dressing) with the watercress and toss. Fold in the avocado, mango and red onion. Sprinkle with Petitas