Sauté mirepoix and mushroom trimmings in butter with veal bones, bay leaves and thyme until bones are brown around the edges.
Deglaze with vinegar; reduce completely. Add wine; reduce by half.
Add veal stock and veal meat trim; bring to a boil. Reduce heat; skim surface. Add thyme. Simmer, skimming frequently, until slightly thickened and reduced by about half. Strain sauce through chinois. Keep warm for immediate use or cool down quickly on ice.
Just before serving, reheat sauce, if necessary. Whisk in truffle butter. Season with salt and pepper.