Make the Roux in the normal manner and allow to cool. Mix the milk into the Roux so as to obtain a smooth sauce and bring to boiling point. Meanwhile, cut the veal into small cubes and stew with the butter without colouring, adding the onions, seasonings and thyme; place into the sauce. Allow to simmer gently for 2 hours and pass through a fine strainer. Coat the surface of the sauce with butter to prevent the formation of a skin.