Peel the vegetable marrow, and cut it into rather thin slices, cut the onion in quarters, and put all into a good-sized saucepan in which the butter has been dissolved; add the salt and water, and simmer for one hour. Strain through a sieve, rubbing as much of the pulp through as possible; return the soup to the saucepan, shake in the semolina, stir for ten minutes after it boils, and add the milk just before serving.