Peel the pumpkin and cut in 3 inch lengths. Halve the tomato, and cut the onion in thin slices. Put all in a thick saucepan with a closely fitting lid. Do not add any liquid of any kind. Put the saucepan in a slow oven or on a very slow fire for 1 hour. Then rub all through a sieve into a basin. Put the purée into a saucepan and dilute with the hot milk. Season with salt, pepper and sugar. Mix the yolks of eggs in a basin, and dilute with a little cold milk, remove the saucepan from the fire, mix a little of the hot soup to the eggs, and then add the eggs to the soup. Stir for a few minutes, and serve with croûtons.