Preheat broiler and place the rack as close as you can get it to the element.Season the flank steak on both sides with the steak seasoning and set aside.
In a skillet over medium heat, melt the butter. Add in the sliced onions and toss until it is all coated with the butter. Cook the onions until they are soft and translucent. You don't want to brown them, just sweat them down. Sprinkle the onions with sugar and toss, then stir in the vinegar and salt. Continue to cook while you cook the flank steak.
Add the flank to the oven and broil on each side for 3-5 minutes. This will differ depending upon the thickness of your steak. You are looking for an internal temperature somewhere around 120 degrees for medium rare. You don't want to cook flank steak much past medium, otherwise it will be a little dry and tough.
Once the steak is cooked on both sides, remove to a cutting board to rest for 5 minutes before thinly slicing across the grain. Top each serving with the onion marmalade.
If you have leftover, then you are in for a treat! this recipe makes and amazing sandwich the next day with a little Monterrey jack cheese on a crusty loaf.