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Bring cranberries, blueberries, wine, syrup, chiles, zest, and 1/4 teaspoon salt to a boil in a 10-inch heavy skillet, covered, over moderate heat. Boil, covered, just until cranberries burst, 4 to 6 minutes, then remove from heat and let stand, covered, 20 minutes. Discard chiles and zest.
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While sauce stands, put oven rack in middle position and preheat oven to 350ºF.
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Mix together pepper and remaining teaspoon salt. Pat buffalo dry and sprinkle all over with salt mixture. Heat oil in a 13- by 9-inch flameproof roasting pan over moderate heat until hot but not smoking, then brown buffalo well on all sides, about 10 minutes total.
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Transfer pan to oven and roast until thermometer inserted diagonally 2 inches into center of meat registers 120ºF, 25 to 30 minutes. Transfer roast to a platter and let stand, loosely covered with foil, 15 minutes. (Meat will continue to cook as it stands, reaching 130°F for medium-rare.)
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Bring sauce to a simmer and swirl in butter and any juices from meat accumulated on platter. Stir in salt and pepper to taste.
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Cut roast across the grain into 1/2-inch-thick slices and serve with sauce.