In a skillet over medium-low heat, melt 2 tablespoons of butter or add a drizzle of olive oil. Once hot, add the thinly sliced onions and cook, stirring occasionally, for about 30-40 minutes until deeply caramelized and golden brown. Stir in 1 teaspoon of balsamic glaze five minutes before they’re done. Set aside.

Remove the New York Strip steak from the fridge and let it sit for about 30 minutes to reach room temperature. Pat dry and season generously with salt and black pepper. Heat a skillet over high heat until very hot, then add a splash of olive oil. Sear the steak for 2-3 minutes on each side for medium-rare, then let it rest for at least 10 minutes before slicing against the grain.

Butter the cut sides of the ciabatta loaf or drizzle with olive oil. Toast the ciabatta, cut side down, in the same hot skillet or under a broiler until golden brown, about 2-4 minutes. Watch closely to prevent burning.

Spread a generous layer of lemon herb aioli on both cut sides of the toasted ciabatta. Layer the sliced steak, followed by the sweet caramelized onions. Drizzle the remaining balsamic glaze over the onions and finish with fresh arugula. Press the sandwich together gently and serve warm.