Put the potatoes in a medium saucepan and cover with hot water. Cook over high heat until tender, about 12 minutes.

Meanwhile, heat the oil in a large, heavy casserole. Add the chicken and cook over high heat until lightly browned, 4 minutes. Reduce heat to medium and add the onion, ginger, garlic, lemongrass and jalapeño. Cook, stirring, until the onion is softened, about 4 minutes. Add 3 tablespoons of the fish sauce and the broth and bring to a boil. Cover and cook over moderate heat until the chicken is just cooked through, about 10 minutes.

Using a slotted spoon, transfer the potatoes to a ricer and press into a bowl. Save the potato water. Add flour, egg yolk, chives and salt to the potatoes and stir until a stiff dough forms (you may need to add additional flour gradually until the dough comes together). Turn the dough out onto a heavily floured board and divide it in half. Roll each piece into a 1/2-inch-thick rope. Cut the ropes into 1-inch pieces.

Add the bok choy to the stew and simmer for another 5 minutes. Add the cornstarch mixture and cook, stirring, until thickened, about 1 minute.

Return the potato water to a boil and add the dumplings. Cook over high heat until they rise to the surface, then simmer for 2 minutes. Using a slotted spoon, transfer to the chicken stew. Add the basil and the remaining 1 tablespoon of fish sauce and simmer the stew for 2 to 3 minutes. Serve with lime wedges and sriracha, if desired.