Place the London Broil steak in a dish and prick with a dinner fork.  Rub each side with salt.

Puree olive oil, garlic, onion and thyme.  Rub the steak with the paste. Cover with plastic wrap and refrigerate overnight.

The next day, simmer the balsamic vinegar (do not boil) until reduced by half.  Add the butter and let it melt.

With a paper towel, wipe the paste off the steak and season both sides with salt and pepper.

Grill until medium-rare.  Slice (thin) and serve with a drizzle of balsamic vinegar glaze.

Use the best Balsamic vinegar that you can afford.  Fig Balsamic vinegar is wonderful in this recipe!