1 1 1/4-pound top sirloin steak, about 3/4 to 1 inch thick
3/4 teaspoon ground black pepper
1/2 teaspoon salt
Melt 2 tablespoons butter in heavy large nonstick skillet over medium-high heat. Add shallots; sauté 2 minutes. Add mushrooms; cover and cook 5 minutes. Uncover and sauté until mushrooms are browned, about 10 minutes. Add flour and stir 1 minute. Add Zinfandel, Port, and broth. Boil until sauce thickens enough to coat spoon thinly, stirring occasionally, about 10 minutes. Season sauce to taste with salt and pepper. Set aside.
Meanwhile, place steak between sheets of waxed paper. Using mallet or rolling pin, pound steak to 1/2-inch thickness. Coat both sides of steak with 3/4 teaspoon pepper and 1/2 teaspoon salt. Melt remaining 1 tablespoon butter in another heavy large skillet over medium-high heat. Add steak to skillet and cook to desired doneness, about 2 minutes per side for medium-rare. Transfer steak to work surface; do not clean skillet. Add mushrooom sauce to skillet; bring to simmer, scraping up any browned bits.
Thinly slice steak; transfer to platter. Spoon mushroom sauce over and serve.