Return the onions to the saucepan and simmer in enough water to cover for 10 minutes, until they are tender. Be careful not to overcook. The onions should still be firm and retain their shape. Drain the onions and leave in the colander for a minute.
In the same saucepan melt the butter. Add the flour and mix well. Stirring constantly, pour the cream in very slowly. Cook the sauce over medium heat, stirring constantly until the sauce thickens. Add the spices and the onions and cook just until the onions are reheated.
Pour into a serving dish and sprinkle with additional nutmeg if desired.