Place tenderlounds side by side and tie together with kitchen string. In a small bowl, combine the mustard, buttermilk, thyme, pepper, and salt; spread over surface of meat. press crumbs onto meat.
Place on a rack in a shallow roasting pan. Cover and bake at 425°F for 15 minutes. Uncover; bake 35-40 minutes longer or until a meat thermometer reads 160°F let stand for 10 minutes. Remove string before slicing.