Ingredients

2 lb London Broil (beef top round) steak

3/4 cup balsamic vinegar, divided

1/4 cup olive oil

1/2 cup spicy brown mustard, divided

2 garlic cloves, minced

1 1/4 tsp salt, divided

1/2 tsp black pepper

1/4 cup (1/2 stick) butter

4 medium-size sweet onions, thinly sliced

Preparation

  1. Place the steak in a large resealable plastic storage bag.

  2. In a medium bowl, combine 1/2 cup vinegar, the oil, 1/4 cup mustard, the garlic, 1/2 tsp salt and the pepper, mix well. Pour over the steak then seal the bag and chill for 1 hour.

  3. 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 vinegar, 1/4 cup mustard and 3/4 tsp salt. Reduce heat to medium-low and cook until the mixture thickens, stirring often.

  4. Preheat the grill to high heat. Grill the steak for 12 to 15 minutes fo medium-rare, or to desired doneness beyond that, turning frequently; baste often with the marinade for only the first 5 minutes or cooking. Thinly slice the steak and serve topped with the sauteed onions.