• Steamed white rice, for serving
Fill a medium-size bowl with cold water. Peel sweet potatoes and give them a good rinse under cold running water. Cut them into 1-inch cubes and submerge them in the bowl of water to prevent them from turning gray.
Slice off root ends of lemon grass stalks and the top three-fourths of the blade-like greens. All you will use is about 3 inches of the light-colored, tightly knotted stalk. Slice these in half lengthwise, then cut each half into thin strips. Stack a few of these strips at a time and cut them crosswise into tiny pieces. (One stalk gives 1 generous tablespoon of finely chopped lemon grass.)
Heat oil in wok or large saucepan over medium-high heat. Once oil appears to shimmer, add chicken cubes and stir-fry to sear them, 1 to 2 minutes.
Drain potatoes in a colander. Add to chicken along with 1 cup water, the lemon grass, salt and peppercorns. Bring to a boil, reduce heat to medium-low, and cook the mixture, covered, stirring occasionally, until potatoes are tender and chicken pieces are cooked through, about 10 minutes. When you cut into a piece of chicken, it should no longer be pink in the middle and the juices will run clear.