Season steak with the 1/2 teaspoon salt and the pepper; sear in a cast-iron skillet over high heat, turning once. Heat oven to 350 degrees F and bake steak for 15 minutes. Let rest 10 minutes.
Meanwhile, cook onions in butter in a large skillet 25 minutes, until golden brown. Raise temperature on onions; add wine and the 1/4 teaspoon salt; cook 2 minutes. Slice steak. Serve with onions. Makes 6 servings.