Prepare the Steak:

• Season the steak: Generously season both sides of the steak with salt and pepper.
• Heat the skillet: In a hot skillet or grill pan, add 1 tbsp olive oil and sear the steak for 2-3 minutes per side for medium-rare. Adjust the cooking time for your desired doneness.
• Rest the steak: Remove the steak from the pan, cover it loosely with foil, and let it rest for 5 minutes before slicing thinly.

Make the Caramelized Onions:

• Cook the onions: In the same skillet, add the remaining olive oil and sliced onions. Cook over medium heat until soft and golden brown, about 8-10 minutes.
• Deglaze with balsamic vinegar: Stir in the balsamic vinegar and cook for another 2 minutes. Set aside.

Prepare the Sauce:

• In a small bowl, mix mayonnaise, Dijon mustard, and minced garlic until smooth. Adjust seasoning with salt and pepper if needed.

Assemble the Sandwich:

• Toast the bread: Lightly toast the baguette or ciabatta roll for added crunch.
• Layer the ingredients: Spread the sauce on both halves of the bread. Add a layer of arugula, tomato slices, and caramelized onions.
• Add the steak: Place the thinly sliced steak on top and close the sandwich.

Serve:

• Slice the sandwich into portions, serve immediately, and enjoy!