To make the sauce, preheat a grill or broiler. Peel the onions and cut into thick wedges. Brush onions, mangos, poblano or Serrano and red peppers, and scallions lightly with olive oil, then grill over a preheated charcoal grill, gas or electric grill. Alternatively, place on a shallow heatproof pan and broil. Cook the vegetables, turning occasionally, until lightly charred.
Remove from the grill, cool, then trim and seed the peppers and cut away the root ends of the onions. Rub off most, but not all, of the blackened skins. Slice the scallions, dice the onions and the peppers and combine with the remaining ingredients and pulse in a blender or robot coupe.
Season the fish with salt and pepper and then brush with olive oil. Saute the fish 3 to 4 minutes, then flip it over and continue sauteing about 3 minutes or until the fish starts to brown around the edges.
Remove the fish then add the Jack Daniels until the alcohol is cooked out, add in the brown sugar and the Criollo Sauce until bubbling hot, arrange the fish on serving plates and spoon the sauce over the edge of the fish.