Place fish and water in a small pan. Bring to a boil, decrease heat, and simmer until fish just begin to flake, 6 to 8 minutes. Remove fish, reserving liquid. Flake fish with a fork and set aside. Soak rice noodles in warm water to cover for 15 minutes; drain. Cook noodles in a large pot of boiling water until tender, 3 to 5 minutes. Drain, rinse, drain again
To make spice paste, thinly slice lemongrass. Place in a blender with shallots, garlic, chilis, salted fish, galangal, shrimp paste, and water; process until smooth
Cook beans and cabbage in boiling water until tender-crisp, 2 minutes. Drain, rinse, and rinse again. In a 3-quart pan, combine spice paste with 2 cups coconut milk. Bring to a boil; cook for about 1 minute. Add reserved fish liquid and remaining 1 cup coconut milk; bring to a boil and cook for 2 minutes. Decrease heat and simmer for 5 minutes more. Add fish, fish sauce, and sugar, cook for 30 seconds
To serve, arrange noodles, beans, cabbage, and bean sprouts on a platter, Ladle coconut mixture over serve