3 Large Onions (if you can find Vidalla's - they are the best)
Salt & Pepper
2 T. Sugar
Marble Rye Bread (or regular rye)
Lettuce & tomato
Extra Virgin Olive Oil
Cut the onions in half then use a mandolin (or your super handy knife skills) to slice the onions into paper thin slices. Heat a tiny amount of the olive oil (or you can even use cooking spray) over medium-high heat. When the oil is hot add the onions, sugar and 1/2 t. thyme. Allow to cook until golden brown and caramelized.
While the onions are cooking:
Pre-heat the oven to Broil or start up your BBQ. Season the steak liberally with salt & pepper (and maybe a dash of cayenne pepper is you like a little heat). Cook the steak for about 7-8 minutes per side for medium doneness. When the steak is cooked, let it rest on the cutting board for at least 5 minutes so the juices don't run when you slice it.
Once the onions are golden add the balsamic vinegar and a dash of salt & pepper. Cook until all the liquid has evaporated.
Slice the lettuce & tomatoes and thinly slice the steak.
Layer the steak, onions, lettuce & tomato on the rye bread that has been slathered with a thin layer of Dijon mustard.
makes 6 hearty sandwiches - depending on size of steak.