Put the butter in a saucepan, and as it melts add the chopped onion, and let it stew until very tender, but do not let it brown. Mince the ham and cut the vegetables very fine, and add first the ham, letting it brown a minute, and then the vegetables, herbs and spices. Let all simmer gently for ten minutes, without browning. Add the Sauce Veloutee, stir in well, and bring all to a boil. Let it boil ten minutes, and be sure to stir constantly. Then add, by gentle degrees, the Cream, which should not be heated, but which must be very rich and sweet (if not perfectly sweet it will spoil the Sauce). When all this is blended, the Sauce is of a velvet smoothness, and very delicious. Strain and set on the fire a minute longer to heat, and serve hot. It is served with fish, chicken and sweetbreads.