Instructions: Place the steak in a large resealable plastic storage bag. In a medium bowl, combine 1/2 cup vinegar, the oil, 1/4 cup mustard, the garlic, 1/2 teaspoon salt, and the pepper; mix well. Pour over the steak, then seal the bag and chill for 1 hour.

Meanwhile, melt the butter in a large skillet over medium-high heat. Add the onions and saute for 12 to 15 minutes, or until very tender, stirring occasionally. Add the remaining 1/4 cup mustard, 1/4 cup vinegar, and 3/4 teaspoon salt. Reduce the heat to medium-low and cook until the mixture thickens, stirring often.

Preheat the grill to high heat. Grill the steak for 12 to 15 minutes for medium-rare, or to desired doneness, turning frequently; baste often with the marinade for only the first 5 minutes of cooking.

Thinly slice the steak and serve topped with the sauteed onions.

This recipe yields 6 servings.

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