Wash and trim lemongrass, only use the thick part of the lemongrass. (note 2)
If using pestle and mortar: finely slice then pound the lemongrass until fragrant. About 1 minute. Add garlic and onion. Pound for a further minute until a rough dry paste is formed.
If using a food processor: blend lemongrass for about 5 seconds. Add garlic and onions, blend for a further 10 seconds.
Finely chop method: if you have neither of the two above tools, simply finely chop the lemongrass, garlic and onions.
Place lemongrass mixture into a large mixing bowl. Add fish sauce, soy sauce, sugar and oil. Mix well then add in chicken thighs and coat chicken well with the marinade. Cover and marinate in the fridge for 1 hour or even better, overnight.
30 minutes before cooking, take the marinated chicken out of the fridge to bring it up to room temperature for even cooking.
Preheat oven grill/broiler to 230°C/450°F.
Place chicken thighs on a foil-lined baking tray and broil/grill for 15 minutes or until chicken is cooked and lightly charred. No need to flip.