I find the best sandwiches are usually those cobbled together from a variety of delicious leftovers. This is why I find it essential to have on hand the proper accompaniments to make an average sandwich truly great: caramelized onions, good cheese, a variety of herbs and a worthy platform.
Pulling together a sandwich recipe from scratch seems a little silly as I would normally be using last night's grilled steak...but here is a how I might go about it from the beginning.
This could be made with any good, thinly sliced steak, but I liked the sirloin flap for it's quick time in a rub and on the grill. It has a good texture, is relatively inexpensive and marries beautifully with the rich, sweet, caramelized onions. An herbed sauce, ripe tomato, peppery greens, a soft pretzel bun and of course some good, melty gouda perfect this sandwich for me. —savorthis
Mix garlic, onion powder, paprika, pepper and 2 T oil. Toss steak to coat and let sit at room temperature 30 minutes or refrigerate up to 8 hours.
Heat remaining oil in a large pan over medium heat and cook onions with a small pinch of salt until beginning to brown. Deglaze pan with 1/3 broth and cook again until brown bits form in pan. Repeat until broth is gone and onions are a deep golden brown (20-30 minutes).
Heat grill to medium high. Season with salt and grill steak to 125° and nicely browned. Let rest ten minutes. Toast cut sides of buns over grill, then flip top and top with cheese, moving to indirect heat until cheese melts.
Mix herbs, mayo, sour cream, sriracha (if using). Cut steak into 3 chunks and slice thinly across the grain. Return slices to plate and allow to soak up any juices.
To assemble: top bottom bun with some herbed sauce, steak, onions, tomato slices, arugula and top bun. Enjoy right away.