Caramelize onions: Melt butter or heat olive oil over medium-low heat, add sliced onions and season with salt and black pepper. Cook gently for 30-40 minutes, then add balsamic glaze in the last 5 minutes.

Cook steak: Bring steak to room temperature, season, and sear in a hot skillet for 2-3 minutes on each side until golden. Let rest for 10-15 minutes, then slice thinly.

Toast ciabatta: Butter or drizzle olive oil on ciabatta cut side. Toast in a hot skillet or under the broiler for 2-3 minutes until golden.

Assemble sandwich: Spread lemon herb aioli on both halves, layer sliced steak, caramelized onions, drizzle with balsamic glaze, top with arugula, and close the sandwich.

Serve warm: Slice in half and serve immediately with crispy French fries or a salad.