Melt the butter in a large saucepan. Add the onion and cook over moderate heat, stirring, until softened, 4 minutes. Add the stock and water and bring to a boil. Add the chopped asparagus and season with 1 1/2 teaspoons of salt and a few grindings of pepper. Boil over moderately high heat until the asparagus are tender, about 10 minutes. Remove from the heat and let stand for 5 minutes.