2 very firm underripe mangoes (about 2 pounds)
1/3 cup Nuoc Cham Dipping Sauce
6 scallions, white and pale-green parts only, thinly sliced on the bias (about 1/3 cup)
1/4 cup lightly packed fresh mint leaves, coarsely chopped (from 1 bunch)
1/2 cup lightly packed fresh cilantro leaves and stems, coarsely chopped (from 1 bunch)
2/3 cup roasted unsalted peanuts, coarsely chopped
Peel mangoes. Cut 1/4-inch-thick slices of flesh away from pits, then cut slices lengthwise into 1/4-inch-thick batons. Gently toss together mangoes, nuoc cham sauce, scallions, and herbs in a large bowl. Let salad stand, stirring occasionally, at least 30 minutes and up to 2 hours. Before serving, stir in peanuts.