To make the dressing for the slaw, combine the green onions, vinegar, honey, chiles, mayonnaise, oil, salt and pepper to taste in a blender and blend until emulsified.
Combine the cabbage, red onion and poppy seeds in a bowl, add the dressing, and stir until combined. Fold in the cilantro and season with salt and pepper to taste. Cover and refrigerate while you grill the chicken or for up to 1 hour.
Heat your grill to high.
Stir together the ancho powder, pasilla powder, cumin, coriander, ginger, brown sugar, garlic powder, onion powder, allspice, cinnamon, cloves, fennel, chile de arbol, 2 tablespoons salt, black pepper and cayenne in a small bowl.
Brush both sides of the breasts with the oil and season with salt. Rub the top side of each breast with a few tablespoons of the rub and place on the grill, rub side down. Grill until golden brown and slightly charred, 3 to 4 minutes. Turn the breasts over and continue grilling until just cooked through, 4 to 5 minutes longer.
Remove the chicken from the grill and drizzle with the black pepper vinegar sauce. Tent loosely with foil and let rest for 5 minutes. Serve the slaw on the side.
BLACK PEPPER VINEGAR SAUCE:.
Combine the vinegar, oil, mustard, honey, salt and pepper in a blender and blend until smooth. The sauce can be made 1 day in advance, covered and refrigerated. Bring to room temperature before serving.