Step 1

Melt the butter in a large skillet over medium-low heat. Add the sliced onion and stir. Cover with a lid and cook for 20 minutes, stirring occasionally. Remove the lid and stir in the sugar, salt and pepper. Cook for a further 10 minutes, until brown a bit and caramelize.

Step 2

Pat the steak dry with paper towels. Generously season it on both sides with salt and pepper. Using a grill or a skillet, cook the steaks to your preferred temperature and let them rest for 10 minutes.

Step 3

Slice the steak into thin strips against the grain. Cut the ciabatta buns in half horizontally.

Step 4

Spread a tablespoon of pesto on each bottom half of the buns, then top with ½ cup each of baby arugula, followed by steak slices, caramelized onions and tomatoes. Spread mustard and mayo on the top bun then place on top.