Melt butter in heavy skillet.
Add onions and shallots; until golden brown.
Add mushrooms and sauté until browned.
Add Madeira wine and continue cooking until mixture is reduced to au sec.
Add veal stock and bring to a boil.
Continue to cook an additional 10 to 15 minutes or until liquid is reduced by one- half.
Whisk in butter, add sugar and season with salt.
Strain sauce and keep warm.