Grilled Flank Steak With Red Onions

1-2 lb. flank steak, season with salt, black pepper¼ c. balsamic vinegar¼ c. olive oil¼ c. lemon juice¼ c. Dijon ¼ c. brown sugar¼ c. garlic puree1 spring fresh rosemary¼ c. fresh parsley, chopped¼ c. fresh basil, chopped2 medium red…