Put the onions in a bowl with the vinegar and soak 10 minutes.
Sprinkle the chicken with 1 teaspoon salt and a few grinds of pepper. Place skin-side up in a large ovensafe skillet and roast until the skin is golden brown and crispy and the chicken is cooked through, about 35 minutes.
Meanwhile, toss the beans, mango and cilantro with the soaked onions and vinegar, 1 tablespoon of the coconut milk and salt and pepper to taste in a medium bowl.
Transfer the chicken to 4 dinner plates. Carefully pour out the excess fat from the skillet and place on the stovetop over medium-high heat (use caution; the handle will be extremely hot). Add the chipotle chiles, remaining 1/2 cup coconut milk and 1/2 cup water; bring to a boil, scraping the browned bits from the pan. Reduce the heat and simmer until the sauce is slightly thickened, about 3 minutes. Add a splash of vinegar and season with salt.
Spoon the sauce over the chicken and serve with the black bean-mango salad on the side.
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