Step 1Whisk together 1/2 cup hot water and sugar in a medium-size heatproof bowl until sugar is dissolved. Stir in vinegar, fish sauce, lime juice, garlic, and Thai chiles. Set aside, or cover and refrigerate until ready to use. Make the salad
Step 2Arrange tomatoes and mangoes on a large platter or plate. Nestle torn mozzarella in and around fruit. Spoon about 1/2 cup vinaigrette over fruit and mozzarella. (Reserve remaining vinaigrette for another use.) Top with basil leaves and serrano chile. Serve slightly chilled or at room temperature.