Preheat a charcoal grill to medium, ash-covered coals or a gas grill to 300° to 350° (medium) heat. If desired, brush roast with oil.
Grill roast, turning occasionally, on a charcoal grill, without grill lid, 35 to 45 minutes, or on a gas grill, covered with grill lid, 25 to 30 minutes.
Meanwhile, bring peppercorns and next 3 ingredients to a boil in a large skillet over medium-high heat. Cook, stirring occasionally, 12 to 14 minutes or until mixture is reduced by half.
Whisk together cornstarch and 2 Tbsp. water until smooth. Add cornstarch mixture and mustard to skillet. Cook, stirring constantly, 2 minutes or until slightly thickened.
Transfer roast to a serving platter, sprinkle with salt and pepper, and let stand 10 minutes. Carve roast into thin slices. Serve with warm peppercorn sauce.
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