Heat a large skillet over medium heat. Add olive oil and onions and cook until tender, about 20 minutes. Remove from skillet.
Season ribeye with salt and black pepper. Turn skillet to high heat and cook for two minutes per side, until medium rare. Remove from the pan and set aside.
Prepare a baking sheet with foil or parchment paper. Place ciabatta buns on the baking sheet. Combine melted butter and garlic powder, then brush on the buns. Top one of the buns with provolone. Bake for five minutes, until golden brown and the cheese is melted.
Combine mayonnaise, grainy mustard, and horseradish in a small bowl.
Assemble the sandwiches by spreading mayonnaise mixture with buns, and topping with ribeye, onions, and arugula. Serve immediately.
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