First prepare the marinade. If you are using fresh lemon grass, cut it crossways into very thin slices, starting at the bulbous bottom end and going up around 6". Discard the strawlike top. If you are using dried lemon grass, soak it in 4 tb of hot water for an hour. Put the chicken pieces in a bowl, add the fresh lemon grass or the drained soaked dried lemon grass (save the soaking liquid). Peel and crush the large garlic clove, peel the ginger and grate it finely. Add the garlic, ginger, sugar, tomato paste, salt, chilli powder and turmeric to the chicken. Mix, cover and set aside for 1-24 hours, refrigerating if necessary. Peel and finely chop the two garlic cloves. Put the oil in a wok or large, lidded frying pan and set over a high heat. When it is hot, put in the garlic. Stir and fry for 30 secs or until the garlic is golden. Add the chicken along with its marinade. Stir and fry for 5-6 mins or until the chicken browns a little. Add the fish sauce and either the lemon grass soaking liquid or 4 tb stock. Stir once and cover. Cook on a high heat for 5 mins. Lift the lid and stir, adding another 4 tb of stock. Cover, turn the heat to low and cook for another 5 mins. While the chicken cooks, peel the onions and cut them into 0.75" dice. Separate the onion layers within the diced pieces. Turn the heat under the chicken to high, remove the wok lid, add the onion and fry for 1 minute. Lift the chicken out its oiul and serve.