Heat the sesame oil in a large wok or pan. Chop the garlic, the chillies and the lemongrass stalk and sauté.
Add the meat and fry vigorously. Remove the meat and slowly heat the curry paste in the pan until it starts to smell. Deglaze with the coconut milk and water. Heat slowly until the paste and milk have combined.
Add the meat and peppers to the milk and simmer for 20 minutes. Add the sugar and fish sauce and simmer for another 5 minutes. If necessary, let it thicken a bit.