This is the best steak sandwich I have ever had. And that is with the beef available in the Philippines.

Heat up some oil in a pan.
Slice up a white onion and cut the slices in half. Throw it into pan and toss with a bit of butter. Cook until soft, separating the rings.
Make garlic butter. Garlic, olive oil, butter, parsley, salt and pepper
I use 35-40mm thick steak. Ribeye if I can get it (rarely); the best available cut if not. Rub with salk/pepper. Grill to medium rare, 55C/130F. Will reach final temp while resting.
Toss some arugula in a bowl with olive oil, salt and pepper.
Mix Sriracha, Asian mayo sauce, and Worcestershire sauce.
Toast each side of two slices of French bread with the garlic butter.
Slather the Sriracha sauce on one of the slices. Top generously with arugula.
Cut steak into slices and pile it on top of the arugula.
Onions go on next. Then slather the other slice with the Sriracha sauce and finish.
I enjoy this with a shot and a Corona.