Steak Sandwich with Caramelized Onions

I don't know how it happened, but all of a sudden my DVR is recording Barefoot Contessa every day. Probably more like 5 episodes a day. I've always loved the simple and delicious recipes of Ina Garten, but tall guy does not appreciate her New England sensibilities and her trips to the "market" for fresh lobster, caviar and truffle oil. So I have to watch them in secret or deal with the snickering in the background.

Anyways, this sandwich is reproduced from one of those episodes. Most of my prep was done in the morning so I could enjoy my day and then put it together at the last minute. I caramelized the onion when it was still cool in the house and then grilled a flank steak right before serving (yes, this girl can BBQ with the big boys).

Steak Sandwich with Caramelized Onions

Saute until brown:
1 large sliced onion
1 T. olive oil
1 tsp. sugar
Set aside or refrigerate until ready to serve

Lightly score flank steak and marinate in: (4 hours+)
2 T. soy
2 T worchestershire sauce
1/4 cup olive oil
2 cloves minced garlic
1 tsp pepper
1 T. brown sugar

Cook on a hot grill until medium rare. Let rest 10 minutes then thinly slice against the grain.

Sauce:
1/4 cup mayo
2 T. brown mustard

Arrange:
4 toasted ciabatta rolls
spread 1 T. sauce on each
add 4 oz. sliced flank steak
then 1/4 of the onions
2 tomato slices (optional)
top with spinach or arugula