Combine pepper, salt, thyme, garlic and mustard; rub all over roast. Place roast, fat side-up, on rack in roasting pan. Insert oven-safe meat thermometer into centre of roast, avoiding fat.
Place in 450°F (250°C) oven for 10 minutes. Reduce heat to 275°F (140°C); cook until thermometer reads 155°F (68°C) for medium, about 2-¼ to 2-¾ hours.
Remove roast to cutting board; cover with foil for 15 minutes, while making the Onion Purée.
Carve roast across the grain into thin slices. Serve with Onion Purée.
Onion Purée: Drain all but 2 tbsp (30 mL) fat from roasting pan (if necessary). Add 2 tbsp (30 mL) olive oil (if necessary), 2 onions (cut into wedges), ½ tsp (2 mL) EACH salt and pepper; cook over medium-high heat until onions are softened, about 5 to 10 minutes., Meanwhile, whisk together 1-½ cups (375 mL) sodium-reduced beef stock with 1 tbsp (15 mL) EACH horseradish and Dijon mustard. Add to roasting pan. Bring to boil, stirring to bring up any browned bits from the bottom of the pan. Reduce heat and simmer until slightly thickened, about 5 minutes. Transfer to blender; blend until applesauce like. Makes 2 cups (500 mL) sauce.