Lime juice, garlic, and fresh mint come together in a lively dual-purpose dressing here. Half is used to marinate the chicken breasts, while the other half is tossed together with watercress, mango, and charred radishes for the salad.
Preheat grill to medium-high. In a bowl, combine lime juice and garlic. Whisk in oil, then chopped mint; season with salt and pepper. Pound chicken to an even 1/2-inch thickness; transfer to a baking dish. Season with salt and pepper; toss with half of dressing. Let stand 10 minutes.
Brush grill grates with oil. Grill chicken, radishes, and serrano, flipping chicken once and turning vegetables occasionally, until chicken is just cooked through and radishes are charred in places, 6 to 8 minutes. Mince serrano; stir into remaining dressing. Serve chicken with radishes, watercress, and mango; drizzle with dressing and top with cheese and mint leaves.