Heat 1 tablespoon of butter in a skillet over medium heat. Add the sliced onions and cook until golden and caramelized, about 10-12 minutes. Remove from skillet and set aside.

Season the sliced steak with salt, pepper, and optional garlic powder or steak seasoning. In the same skillet, cook the steak over medium-high heat for 2-3 minutes until browned but still tender.

Butter one side of each bread slice. On the unbuttered side, layer mozzarella and provolone cheese, followed by cooked steak and caramelized onions. Top with another layer of cheese and close with a second bread slice, buttered side up.

Heat a clean skillet over medium heat. Place the sandwich and press slightly. Grill each side for 3-4 minutes until golden brown and the cheese is melted. Flip and repeat.

Slice and serve hot with your favorite sides or dipping sauces.