This one comes from KraftRecipes.com. It hits several of my buttons: steak, onions, balsamic, and easy. Yum.
¼ C balsamic vinaigrette dressing
3 T bottled steak sauce, divided
2 T brown sugar
2 cloves garlic, minced
1 boneless beef sirloin steak* (1 lb)
1 T olive oil
1 large onion, thinly sliced
Mix dressing, 2 T steak sauce, sugar and garlic until well blended. Pour over steak in shallow dish; turn steak to coat both sides. Refrigerate 1 hour to marinate.
Heat oil in large nonstick skillet on medium-high heat. Add onions; cook and stir 5 minutes. Cover; cook on medium-low heat 15 minutes or until onions are golden brown, stirring occasionally.
Meanwhile, heat broiler. Remove steak from marinade; discard marinade. Place steak on broiler pan. Broil steak, 4" from heat, 8 minutes on each side or until done (160°F). Remove from heat; cover. Let stand 5 minutes before cutting across the grain into thin slices. Stir remaining steak sauce into onions. Serve with the meat.
* You can substitute a flank steak if you reduce the cooking time to 6 minutes/side.
4 servings