1. Preheat grill to medium high. Clean grates and then brush with oil.
2. Place onion slices on a cookie sheet or large plate and brush on both sides with oil. Sprinkle 1 tsp (5 mL) Vermont Castings Chophouse Steak Rub on both sides of onion slices.
3. Place onions on grill and cook, flipping once, until soft ened and lightly charred, 5-7 minutes. Remove to a carving board and allow to cool.
4. Combine balsamic vinegar and brown sugar in a small saucepan over medium heat and cook until liquid is reduced by half, 5-7 minutes. Finely chop the cooled onions and add to balsamic mixture, then remove from heat and keep warm.
5. Rub flank steak on all sides with remaining Vermont Castings Chophouse Steak Rub.
6. Grill steak, flipping once, to medium rare, 125F-130˚F (52-55˚C) on an instant read thermometer, 8-12 minutes.
7. Baste meat with the balsamic mixture and cook 1 minute longer, flipping once, to caramelize the sauce.
8. Remove to the carving board and rest 5 minutes before thinly slicing the steak across the grain.
9. Serve with remaining balsamic glaze.