Heat the oil in a high-walled pan. Fry the garlic, ginger, lemongrass, chilli and curry powder in it for 3 minutes over a medium heat, stirring constantly. Add the chicken pieces, onions, sugar and salt and mix gently. Cover and simmer for 8 minutes until the onions are translucent, then mix vigorously so that the chicken is evenly covered with the curry mixture and simmer again for 15 minutes. Pour in coconut milk, bring to the boil, stirring occasionally. Reduce the heat and simmer without the lid for 30 minutes, until the chicken is very tender. Serve garnished with peanuts.