In a small bowl, mix to combine 1 tablespoon olive oil, mustard, garlic, 3/4 teaspoon kosher salt and pepper. Spread mixture all over roast and place on a rimmed baking sheet. Let rest 20 minutes on counter.
In a medium mixing bowl, toss to combine the onion, 1 teaspoon olive oil and 1/4 teaspoon salt. Place around roast on baking sheet.
Preheat oven to 425°. Place roast in oven for 10 minutes. Reduce heat to 325° and cook for 10 minutes, or until internal temperature reaches 125°, for medium rare. Let rest 10 minutes with onions, loosely tented with aluminum foil. Serve on toasted buns.
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