In a large skillet over medium-high heat, cook and stir flour for 4-5 minutes or until light tan in color. Immediately transfer to a small bowl; whisk in broth until smooth. Set aside.
Sprinkle beef with salt and pepper. In the same skillet, cook beef in oil over medium heat for 5-6 minutes on each side or until meat reaches desired doneness (for medium-rare, a meat thermometer should read 145°; medium, 160°; well-done, 170°). Remove and keep warm.
In the same skillet, saute mushrooms until tender. Add the garlic, rosemary and salt; saute 1 minute longer. Stir in the sherry. Stir flour mixture; add to the pan. Bring to a boil; cook and stir for 1 minute or until thickened. Stir in butter until melted. Serve with steak. Yield: 4 servings.