Preheat oven to 325°.

Trim fat from roast. Combine mustard and buttermilk. Spread mustard mixture over roast.

Combine breadcrumbs and next 4 ingredients; press breadcrumb mixture evenly onto roast. Place roast on a rack in a roasting pan coated with cooking spray. Insert a meat thermometer into thickest part of roast, if desired. Bake at 325° for 2 hours or until meat thermometer registers 160°. Let stand 10 minutes before slicing. Garnish with thyme sprigs, if desired.