Brush mangos lightly with oil. Place mangos and poblano peppers in center of grid over medium, ash-covered coals. Grill peppers, covered, 9 to 10 minutes (gas grill times remain the same) or until skins are completely blackened, turning occasionally. Grill mangos 8 to 14 minutes (gas grill times remain the same) or until very tender, turning occasionally. Place peppers in food-safe plastic bag; close bag. Let stand 15 minutes. Set mangos aside.
Press black pepper evenly onto beef steak. Brush onion slices lightly with oil. Place steak in center of grid over medium, ash-covered coals; arrange onion slices around steak. Grill steak and onion, covered, 11 to 15 minutes (over medium heat on preheated gas grill, 13 to 16 minutes) or until steak is medium rare (145°F) to medium (160°F) doneness and onion is tender, turning occasionally. Keep warm.
Meanwhile, prepare Vinaigrette. Cut grilled mangos into 3/4-inch pieces. Combine 1/2 cup mango, lime juice, water and serrano peppers in food processor container. Cover; process until smooth. With motor running, slowly add oil through opening in cover, processing until well blended. Season with salt, as desired. Set aside.
Remove and discard skins, stems and seeds from poblano peppers; cut into 3/4-inch pieces. Carve steak into slices. Cut onion slices in half. Place beef, remaining mango pieces, onion, peppers and radishes on serving platter. Season with salt, as desired. Drizzle salad with vinaigrette; toss lightly to coat. Sprinkle with cilantro.
Nutrition information per serving: 398 Calories; 144 kcal Calories from fat; 16 g Total Fat; 3 g Saturated Fat; 10 g Monounsaturated Fat; 49 mg Cholesterol; 84 mg Sodium; 37 g Total Carbohydrate; 6 g Dietary Fiber; 29 g Protein; 3.2 mg Iron; 9.1 mg NE Niacin; 1.2 mg Vitamin B6; 1.5 mcg Vitamin B12; 5.3 mg Zinc; 32.2 mcg Selenium;