Peel the onions leaving them whole. Place in a saucepan, cover with cold water and bring to the boil. Simmer for 1 minute, then drain and place in a casserole. Add the butter and water. Cover with a lid, place in the centre of a moderate oven (180°C., 350°F., Gas Mark 4) and bake for 1 hour.
About 10 minutes before serving melt the butter for the sauce in a pan. Stir in the flour and cook gently for 1 minute. Gradually beat in the milk stirring well to get a smooth sauce. Season with salt and pepper and simmer gently for a few minutes. When the onions are ready, remove the lid from the casserole and pour over the sauce before serving.