Cut, peel and chop the mango. Place into a food processor with the garlic, ginger lemongrass, Sriracha, lime juice, fish sauce, sugar and salt. Process until smooth. Transfer to a bowl or snap lock bag.
Add the chicken to the marinade and mix well to ensure it is well coated. Cover and refrigerate for at least 2 hours, or overnight if time permits.
Preheat a chargrill or barbecue grill to medium heat. Remove the chicken from the marinade and spray both sides lightly with oil. Cook for about 8-10 minutes on each side or until cooked through.
Meanwhile, use a mandolin or potato peeler to thinly slice the sweet potato into thin rounds or long strips. Pat dry with paper towel. Half fill a medium saucepan with oil and heat over medium until hot. Cook the sweet potato in batches, for 3 minutes until light golden. Use a slotted spoon to remove to a wire rack to cool.