The lemongrass, ginger and onion paste. Place lemongrass into the bowl of the food processor and process until the lemongrass is transformed into a soft, fluffy mass, about 3 minutes. (You'll see a whirling snow blizzard in the bowl.) Add the ginger to the bowl of the food process and pulse until the ginger is finely chopped. Add the onion and process the entire mixture to a nice paste.
Cook the curry. Heat the oil in a large saucepan (4 or 5-quart) over medium heat. Saute the paste of lemongrass, ginger and onion until it is fragrant, about 3 minutes. Add the curry powder and dried red chili flakes (if using) and continue sautéing to aromatize the spices, about 1 minute. Add the chicken pieces, salt, pepper, coconut milk and enough water to just cover the chicken. Bring to a simmer and cook uncovered for 20 minutes.
Add the potatoes. While the curry simmers, peel the potatoes and cut them into 1-inch chunks. Add the potatoes to the curry when the initial cooking time has elapsed. Bring the curry back to a simmer and continue cooking for 15 minutes, until the potatoes are tender. Turn off the heat and let the curry sit, covered, for 30 to 45 minutes to let the flavors meld and mature.
Final touches. Reheat over low heat and correct the seasoning. Serve with lots of steamed rice, or warm baguette for dipping. Lime wedges are a good accompaniment for folks who want to cut some of the heat!