Vietnamese Mango and Chicken Salad

Name: Vietnamese Mango and Chicken Salad
Ingredients: For the salad:; 2 boneless, skinless chicken breasts, about 3/4 pound; 1 tablespoon peanut oil, plus more for brushing chicken breasts; 1 teaspoon kosher salt; 1/3 cup lightly packed mint leaves; 1/3 cup lightly packed Thai or regular basil leaves; 1/4 cup lightly packed rau ram leaves; 2 firm (unripe) mangoes; 1/4 to 1/3 cups chopped roasted peanuts, divided; Additional mint, basil, and/or rau rum for garnish (optional); For the dressing:; 2 tablespoons minced shallot; 3 tablespoons lime juice; 1 tablespoon fish sauce; 1 tablespoon palm or brown sugar; 1 Thai red chile, seeded and finely minced; 1/4 teaspoon freshly ground black pepper, plus more to taste; 1/4 teaspoon kosher salt, plus more to taste; 1 large grapefruit