Prep the Steak: Remove steaks from the fridge 30–60 minutes before cooking. Season generously with salt and black pepper.
Cook the Steak: Heat olive oil in a skillet over medium-high heat. Place steaks fat side down first to render, then cook 2–3 minutes per side until desired doneness. Add butter in the last minute and baste if using. Remove and rest for 5 minutes, then slice thinly.
Caramelize the Onions: In the same skillet, add sliced onions with a pinch of salt. Cook over medium heat for 10–15 minutes until soft and golden. Stir in balsamic glaze and cook another 1–2 minutes until caramelized. Set aside.
Toast the Bread: Slice ciabatta and toast in a pan or toaster until golden and crisp.
Assemble the Sandwich: Spread lemon herb aioli on the bottom half of the bread. Layer with arugula, sliced steak, caramelized onions, and a drizzle of balsamic glaze. Top with remaining bread.
Serve: Slice in half and serve warm.