Rub cut sides of garlic over each side of steaks; sprinkle with 1/4 teaspoon salt and 1/2 teaspoon pepper. Let stand 15 minutes.
Combine port, broth, and rosemary in a large nonstick skillet over medium-high heat; bring to a boil. Reduce heat; simmer, uncovered, 5 minutes or until mixture measures 1/3 cup. Stir 1/4 teaspoon salt into sauce. Pour into a small bowl; set aside. Wipe pan clean with a paper towel.
Coat pan with cooking spray; place over medium-high heat. Add beef; cook 4 minutes on each side or until desired degree of doneness. Set beef aside. Add reserved port mixture to hot pan. Simmer 30 seconds, scraping pan to loosen browned bits, until mixture reduces to 1/4 cup. Remove from heat and stir in butter. Spoon sauce over steaks.