Freeze flat iron steak for 15-20 minutes to firm up for thin slicing
Slice the steak as thin as possible against the grain, holding knife at an angle
Place sliced meat in a mixing bowl and season with kosher salt, black pepper, onion powder, and garlic powder
Warm beef stock in a pot with parmesan rind, thyme sprigs, and optional MSG
Caramelize onions over low heat, deglazing with water as brown fond builds, then add sherry
Heat olive oil in a pan over medium-high heat
Add seasoned meat in a single layer and leave undisturbed for a couple minutes to form a crust
Stir the meat and cook for another 1-2 minutes until lightly cooked
Toast bread and rub with raw garlic clove
Assemble sandwich with toasted bread, caramelized onions, thin sliced steak, and cheese
Wrap sandwich in aluminum foil and warm in oven for a few minutes
Serve with warm beef jus for dipping
