Prepare the Steak: Remove the steak from the fridge and let it sit at room temperature for 20 minutes. Season both sides with salt and black pepper.

Sear the Steak: Heat a non-stick skillet over medium-high heat, sear the steak for 2-3 minutes on each side until golden brown. Add butter while turning. Let rest for 5 minutes before slicing.

Caramelize Onions: In the same skillet, reduce heat to medium, add sliced onions with a sprinkle of salt. Cook for 10-15 minutes until caramelized. Drizzle in balsamic glaze toward the end.

Toast the Bread: Slice the ciabatta in half, toast cut side up under the broiler for 2-3 minutes until golden.

Assemble the Sandwich: Spread aioli on toasted ciabatta, layer with arugula, sliced steak, and caramelized onions. Drizzle with balsamic glaze before sealing.

Serve: Slice the sandwich in half and serve warm, ensuring to share!