Brush mangos with oil. Place mango cheeks and poblanos on grill over medium, ash-covered coals. Grill poblanos, uncovered, 9-10 minutes (gas grill times remain the same) or until skins are blackened, turning occasionally. Grill mangos 8-14 minutes (gas grill times remain the same) or until very tender, turning occasionally. Seal peppers in food-safe bag and let stand for 15 minutes. Cool and cut mangos into 3/4-inch cubes and reserve. Press black pepper onto beef steak. Brush onion with oil. Place steak on grid; arrange onion around steak. Grill, uncovered, 17-21 minutes (over medium heat on preheated gas grill, covered, 13-16 minutes) or until steak is medium rare (145 degrees) to medium (160) doneness and onion is tender, turning occasionally.
For vinaigrette, measure 1/2 cup cubed mango and combine with lime juice, water and serranos in food processor. Cover; process until smooth. With motor running, slowly add oil, processing until well blended. Season with salt as desired.
Discard skins, stems and seeds from poblanos; cut into 3/4-inch pieces. Carve steak into slices. Halve onion slices.Place beef, remaining mango, onion, poblanos and radishes on platter. Season with salt. Drizzle with vinaigrette; toss to coat. Sprinkle with cilantro.