Bring a large saucepan of water to a boil over high heat. Add noodles and return water to boil. Boil until soft, about 8 minutes. Drain and reserve noodles.
Bring chicken stock, fish sauce, garlic, ginger, lemon grass, and green onions to a boil in a large pot. Reduce to a simmer; cook for 10 minutes. Stir in the chicken, bean sprouts, and bok choy. Cook pho until heated through, about 5 minutes.
Divide the cooked noodles between 2 large bowls. Pour pho over noodles; serve immediately.
