In a large resealable bag, combine meat with garlic, balsamic and ¼ cup olive oil. Marinate in the refrigerator for 3 to 6 hours.

Preheat oven to 400°F. Combine onions, shallots and remaining 1 tablespoon olive oil on a large sheet pan and roast 40 to 50 minutes or until tender.

Meanwhile, remove meat from refrigerator for 30 minutes before cooking. Season meat with salt and pepper all over. Heat a large ovenproof skillet over medium high heat and sear London broil 5 minutes per side or until nicely browned. Transfer meat to oven and cook for about 12 to 18 minutes or until a thermometer reads 130°F for medium rare. Let rest 5 minutes before slicing.

To serve place sliced roast on a large platter and garnish with roasted onions.