1. Preheat oven to 400°. toss potatoes with 1 tablespoon oil and rosemary on a rimmed baking sheet; season with 1 teaspoon salt and 1/4 teaspoon pepper. roast 10 minutes. (potatoes will roast 45-60 minutes total.)
2. Meanwhile, finely chop garlic; sprinkle with 1/2 teaspoon salt. using flat edge of knife, smash salt and garlic to form a paste. transfer to a bowl; combine with 1 teaspoon oil and 1/4 teaspoon pepper. rub beef with mixture.
3. Heat a large skillet over high heat. add beef to skillet, turning occasionally, until browned all over, 5-8 minutes. remove baking sheet from oven; nestle beef among potatoes. (reserve skillet.) roast beef with potatoes until internal temperature reaches 125°, about 30-35 minutes. transfer roast to a cutting board to rest; continue roasting potatoes until tender, about 10 minutes.
4. Return reserved skillet to heat; add broth, stirring to loosen browned bits. simmer until reduced by half, 5-8 minutes. Whisk together cornstarch and red wine; add to skillet. simmer until thickened, 2 minutes. remove from heat and swirl in butter; season with remaining 1/4 teaspoon salt and 1⁄8 teaspoon pepper. slice roast and serve with potatoes and sauce.
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