In a large nonstick skillet, heat 1 tablespoon of the oil over medium-high heat. Season both sides of the steak with 1/4 teaspoon of the salt and 1/8 teaspoon of the black pepper. Saute2 to 3 minutes per side for medium-rare. Set aside.

Add remaining tablespoon oil to skillet. Add mushrooms and garlic and cook, stirring occasionally, for 5 minutes. Add flour and cook for 1 minute. Add sherry; gradually stir in broth. Bring to a boil; boil for 1 minute. Reduce heat to medium-low and cook for 3 to 4 minutes or until thickened. Season with the remaining salt and pepper. Stir in butter.

Meanwhile, microwave mashed potatoes following package directions. Serve with sliced steak and sauce.