Prepare all the ingredients before cooking. Make sure the flap steak is cut into thin strips, and slice the bell peppers and onions into thin strips as well.

Season the steak generously with Thundering Longhorn Beef Rub or your preferred beef seasoning. Rub the seasoning into the meat using your hands, making sure it's evenly distributed.

Cut the ciabatta buns in half and set them aside.

Preheat a griddle or large skillet over high heat. You want the griddle to be hot before adding the ingredients.

Once the griddle is hot, add the sliced bell peppers and onions to the griddle. Sauté them for about 5 minutes until they are slightly softened but still have a bit of crunch to them. You don't want them to be too soft.

Season the veggies with salt and pepper to taste. Remove them from the griddle and set them aside on a plate or in a bowl.

Add the seasoned flap steak to the griddle and cook it for about 2-3 minutes on each side until it's nicely seared. You want the steak to be browned and slightly crispy on the outside, but still juicy and tender on the inside.

Once the steak is seared, add the sautéed veggies back onto the griddle with the steak. Mix everything together for a few minutes to allow the flavors to meld together.

Divide the steak and veggie mixture into 4 equal piles on the griddle. Each pile should be big enough to fit on a ciabatta bun.

Top each pile with 3 slices of Pepper Jack cheese. Allow the cheese to melt over the steak and veggies for about 1-2 minutes.

While the cheese is melting, toast the ciabatta buns. You can do this on the griddle or in a toaster oven.

To assemble each sandwich, spread a generous amount of mayo on the bottom half of each bun. You can use regular mayo or flavored mayo if you prefer.

Add a pile of the steak and veggie mixture on top of the mayo. Make sure to include some of the melted cheese in each sandwich.

Finish each sandwich with a generous spoonful of guacamole salsa on top of the steak mixture.

Top each sandwich with the other half of the ciabatta bun, and serve hot.