Put the lemongrass, garlic and onion into a food processor and grind into a paste. Mix in the rest of the ingredients to make the marinade. Taste and correct the seasoning.
In a large bowl, combine the chicken and the marinade and mix well to coat all the pieces. Cover and refrigerated overnight.
If using wooden skewers, soak them in water for an hour so that they don't burn in the oven.
Heat the oven broiler to 220℃.
Drain the chicken from the marinade and thread them snugly on the skewers (I needed 3 skewers), leaving some space at the top and bottom.
Place the skewers on a sheet pan with adequate space between them for heat circulation. Drizzle or spray with oil.
Grill/broil for 15 minutes on one side, turn over and grill for a further 15 minutes till the chicken pieces are cooked and browned. Watch the skewers from time to time so that they don't burn, since ovens can have heat spots. Serve with the raw mango relish.