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Heat oven to 450°G (230°C).
Rub tenderloin with olive oil and place on a roasting pan previously lined with aluminum foil. Season with salt and pepper. Roast beef in the oven for approximately 25 minutes for rare, and 30 minutes for medium rare.
Remove from the oven and wrap in the aluminum foil that lies in the pan. Let beef rest wrapped for 10 – 15 minutes.
While beef is in the oven, prepare the sauce.
Melt half of the butter in a skillet and add sliced onions. Cook onions, stirring occasionally, until they are soft and light caramel color ( 8 – 10 minutes). Add wine and rosemary sprig and bring to a boil until mixture is reduced to ¾ cup. Add beef broth and continue cooking until reduced to 1 ½ cup. Lower heat, remove the rosemary sprig and add the chopped rosemary and sugar. Dice remaining cold butter and gradually add into the sauce, stirring after each addition, until butter is melted. Season to taste.
Unwrap beef tenderloin and pour cooking juices into the sauce. Correct seasoning.
Slice the meat and serve with the sauce.
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