Pomegranate Glazed Flank Steak

2 cups bottled pomegranate juice (16 fl oz)
1 teaspoon sugar
1 teaspoon salt
1 tablespoon sumac
1 teaspoon black pepper
2 pounds flank steak
3 tablespoons unsalted butter
3 tablespoons finely chopped shallots (1 medium shallot)
1/4 cup ruby or tawny port
1 teaspoon fresh lemon juice
Bring pomegranate juice, sugar, and 1/4 teaspoon salt to a boil in a 1 ½ – to 2 quart saucepan over moderately high heat.
Boil until reduced to 1/3 cup (about 20 – 30 minutes)
Preheat broiler
Stir together sumac, pepper and remaining 3/4 teaspoon salt
Transfer steak to baking pan and sprinkle evenly with sumac mixture then let stand about 10 minutes
Broil steaks 3 – 4 inches from heat, turning over once (2 – 3 minutes total for thinner pieces; 3 – 4 minutes for thicker pieces for medium-rare)
Transfer steaks with any pan juices to a large plate and let stand, loosely covered with foil, for 10 minutes.
While steak stands heat 1 tablespoon butter in a 10 inch heavy skillet over moderately high heat until foam subsides. Cook shallots, stirring occasionally, until golden, for 3 – 5 minutes.
Add port and simmer until reduced to a glaze, for 2 – 3 minutes.
Add meat juices accumulated on plate and bring to a simmer.
Whisk in pomegranate reduction and lemon juice.
Remove from heat and whisk in remaining 2 tablespoons butter until incorporated.
Holding knife at a 45-degree angle, thinly slice steak diagonally against the meats grain and serve with sauce.