Prepare garlic bread according to package directions.

Meanwhile, in a skillet over medium heat, sauté onions in three tablespoons oil until translucent. Add mushrooms and sauté for several more minutes until soft. Add salt and pepper to taste.

Raise heat to high and add white wine and deglaze the pan. Turn off heat. Add more salt and pepper if needed. Set relish aside.

Cut skirt steak into four to five equal pieces for easier handling. Coat skirt steak with garlic, black pepper, and red pepper flakes if using.

Using the same skillet (wipe it down in between steps), add remaining three tablespoons oil and heat to high. When oil is very hot, add steak and sauté for two minutes per side for medium rare. Allow meat to rest for 15 minutes.

To assemble the sandwiches: Spread onion porcini relish down one side of the bread. Place skirt steak on bread and slice on a diagonal. For open faced sandwiches, slice garlic bread in half. Top with a piece of skirt steak, porcini onion relish, arugula, and chopped hazelnuts.