In nonstick 12-inch (30-cm) skillet, heat two tablespoons oil over medium heat. Add onions and cook, stirring occasionally, until tender, about 15–20 minutes.

Stir in vinegar, wine, brown sugar, and 1/2 teaspoon salt. Reduce heat and simmer for five minutes. When finished cooking, spoon the red onion marmalade into a small bowl and set aside.

Wash skillet and wipe dry. Sprinkle meat with pepper and remaining 1/2 teaspoon salt.

In skillet over medium-high heat, heat the remaining tablespoon of oil. Add steak and cook six to eight minutes per side for medium-rare or until desired doneness.

Slice steak and serve with red onion marmalade.