Melt the butter in a saucepan. Stir in the flour, and cook over low heat for 3 to 4 minutes. Do not brown. In another saucepan, bring the cream just to a boil. Stir the warm cream into the flour mixture, whipping until smooth. Add the onions and bay leaves, and simmer for 20 minutes on low heat. Season with salt, pepper, and nutmeg. Strain the sauce and serve. Makes 2 cups.