For the marinade: Thinly slice about 6 inches of the lemongrass, starting from the bottom. Mince or press the garlic clove and grate the ginger. Mix all ingredients in a bowl, cover, and refrigerate for 1 hour to 1 day. When cooking: Mince the garlic and chop the onion into 3/4 inch dice. Heat the oil over high heat and add the garlic. Fry for 30 seconds or until garlic is golden, then add the chicken and marinade. Cook for a further 6 minutes or until the chicken starts to brown. Add ¼ cup of stock and some salt (probably half a teaspoon to a teaspoon), stir, and cover. Cook on high for five minutes, then stir, add the rest of the stock, and cook covered over low heat for five more minutes. Turn the heat back up to high, add the onion, and cook for one minute.