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.
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