Heat oven to 400 degrees. Melt the butter in a small saucepan. Add the balsamic vinegar, rosemary, salt and pepper and bring to a boil. Pour into a buttered 2 quart ovenproof casserole. Add the quartered Vidalia onions. Toss to coat and bake 40 minutes, turning and basting twice during cooking.
Season steak generously with salt and pepper. Heat 12 inch cast iron skillet over high heat. Add canola oil and swirl to coat bottom of pan. Add steak and sear for 4 minutes. Turn steak over and place pan in oven with onions. Cook 5 minutes for rare (9 minutes total cooking time). Serve with the balsamic baked Vidalia onions.