Bring pomegranate juice, sugar, and 1/4 teaspoon salt to a boil in a 1 1/2- to 2-quart saucepan over moderately high heat, then boil until reduced to about 1/3 cup, 20 to 30 minutes.
Preheat broiler.
Meanwhile, stir together sumac, pepper, and remaining 3/4 teaspoon salt.
Cut steak into pieces to fit in a large shallow baking pan and pat dry.
Transfer steak to baking pan and sprinkle evenly with sumac mixture, then let stand about 10 minutes.
Broil steaks 3 to 4 inches from heat, turning over once, 2 to 3 minutes total for thinner pieces, 3 to 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, 10 minutes.
While steak stands, heat 1 tablespoon butter in a 10-inch heavy skillet over moderately high heat until foam subsides, then cook shallot, stirring occasionally, until golden, 3 to 5 minutes.
Add Port and simmer until reduced to a glaze, 2 to 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 and serve with sauce.
