I was craving steak and a sandwich, so I made these flank steak, caramelized onion, mushroom, and blue cheese paninis. My son and I had blue cheese on our sandwiches, my husband opted for Swiss, while my daughter didn’t want any at all. With or without cheese, these steak sandwiches were tasty and loaded with flavor. We all devoured our panini and thought they paired nicely with potato salad and a tomato avocado salad.

Flank Steak, Caramelized Onion, Mushroom, and Blue Cheese Panini

Flank Steak Marinade:

1 lb. flank steak
½ cup balsamic vinegar
¼ cup olive oil
1 tbsp Dijon mustard
1 tsp brown sugar
3 cloves of garlic, minced
Sea salt and freshly cracked pepper, to taste

Roasted Garlic Mayonnaise:

1 bulb of roasted garlic
¼ cup good mayonnaise
Sea salt and freshly cracked pepper, to taste

Panini:

2 tbsp butter or olive oil (divided)
1 yellow onion, sliced thick
15 button mushrooms
1 clove of garlic
Thin slices of good bread
Marinated flank steak
Blue cheese crumbles, to taste ( Swiss or Cheddar slices also work, if preferred)

How to Make a Flank Steak, Caramelized Onion, Mushroom, and Blue Cheese Panini:

Prepare the marinade by combining all ingredients in a large zip-lock bag and mixing thoroughly. Add the flank steak and marinate in the refrigerator for 2-8 hours.

Cook the roasted garlic. (Click link for instructions).

Prepare the roasted garlic aioli by combining the mayonnaise, cooled roasted garlic (squeezed from the skin), sea salt, and freshly cracked pepper to taste. Mix thoroughly. Refrigerate until needed to allow the flavors time to mingle.

Caramelized the onion by heating half of the butter or olive oil in a large skillet over medium heat, then adding the onion slices. Cook, stirring occasionally, for 20-30 minutes or until golden brown. Season with sea salt to taste, then remove from heat and set aside. Side Note: If the onions run dry while cooking, add a couple of tablespoons of water, as needed.

Cook the steak by heating a grill pan coated with olive oil cooking spray over medium-high heat. Cook the flank steak for 5-7 minutes per side for medium-rare, or to your desired degree of doneness.

Let the meat “rest” for at least 5 minutes before slicing the steak against the grain into thin slices.

Cook the mushrooms while the steak grills by heating the remaining butter or olive oil in the same large skillet over medium-high heat. Add the mushrooms and leave them alone for 3-5 minutes. Stir them with a spatula and cook for an additional 3-4 minutes or until golden brown. Add the minced garlic and season with sea salt to taste, cook for an additional 60 seconds while stirring constantly. Remove from heat and set aside.

Make the panini sandwiches by preheating the panini maker.

Slather the roasted garlic mayonnaise on each slice of bread, layer with thinly sliced flank steak, caramelized onions, mushrooms, and cheese of choice. Top with the remaining slice of bread. Side Note: You can also add butter to the bread before toasting, but I prefer it without.

Cook the sandwich by placing it on the panini maker and cooking for 5 minutes. Side Note: You can also cook the sandwich for 4-5 minutes on each side in a large skillet or cast-iron skillet.

Slice in half and serve. Enjoy.