Prepare the onions by soaking them in the buttermilk mixture and refrigerate.
While the onions are marinating, caramelize the onions in a skillet and season the ribeye steak with salt and pepper. Place the seasoned steak on a smoker preheated to 250 degrees Fahrenheit. Smoke the steak until it reaches an internal temperature of 120 degrees Fahrenheit.
While the steak is smoking, prepare the steak sauce by combining all the ingredients in a bowl and setting it aside.
Once the onions are caramelized, transfer them to aluminum foil to keep them warm.
After the steak has finishedsmoking, set up the grill for direct heat searing. Open the lid and let the grill heat up for about 8 minutes.
Sear the smoked steak on the direct heat side of the grill for 60-90 seconds per side to achieve a delicious crust.
Remove the steak from the grill and let it rest for 8-10 minutes to allow the juices to redistribute.
While the steak is resting, bread the onions by coating them well in the prepared flour mixture.
Heat oil in a deep fryer or a pot to 350 degrees Fahrenheit. Fry the breaded onions for 3-4 minutes or until they turn golden brown and crispy. Transfer them to a bowl once done.
Toast the ciabatta bread slices until lightly golden.
Thinly slice the rested steak into thin strips, suitable for the sandwich.
Assemble the sandwich by spreading the steak sauce on both the top and bottom buns. Layer the thinly sliced steak on the bottom bun, followed by the caramelized onions. Sprinkle shredded Gruyère cheese on top and melt it using a torch or by placing the assembled sandwich under the broiler until the cheese is melted and bubbly.
Finally, top the sandwich with the crispy fried onions and place the top bun.
Serve the Best Steak Sandwich immediately and enjoy!