Divide the chicken into neat small joints. Melt the butter in a saucepan, add the flour and onion, mixing them in smoothly; cook these for a lew minutes; then add the milk, and stir it over the fire until it boils. Season this sauce carefully. Put in the chicken, and let it heat through very gently. Beat up the yolk of egg with a tablespoonful of milk, and stir it into the sauce. Re-heat it again without letting it actually boil; then serve it very hot.