Prepare 1 envelope Wish-Bone® House Italian Dressing & Seasoning Mix according to package directions. Reserve 1/4 cup prepared Dressing & Seasoning Mix; refrigerate remaining for another use.
Pour 1/4 cup reserved prepared Dressing & Seasoning Mix over steak in shallow nonaluminum baking dish or large resealable plastic bag. Cover, or close bag, and marinate in refrigerator at least 30 minutes.
Meanwhile, preheat oven to 400°. Toss onions and mushrooms with olive oil in shallow roasting pan, then sprinkle with remaining envelope Dressing & Seasoning Mix. Toss to coat. Roast, uncovered, stirring once, 45 minutes or until golden brown and tender.
Remove steak from marinade, discarding marinade. Grill or broil steak until desired doneness. Let stand 10 minutes, then thinly slice. Serve with roasted onions and mushrooms.