Make the Garlic Mayo: Start by mixing mayonnaise, minced garlic, salt, and pepper in a small bowl. Stir until smooth and set aside so the flavors blend while you cook.
Caramelize the Onions: Heat a wide pan over medium heat and add butter. Once melted, add sliced onions and stir gently. Cook slowly for about 15 minutes, stirring often, until soft and golden.
Sweeten the Onions: Sprinkle brown sugar over the onions and stir well. Add balsamic vinegar and let everything cook for another 3 to 5 minutes until glossy and deeply flavored. Set aside.
Season the Steak: Pat the steak dry with paper towels. Season generously on both sides with salt and pepper.
Cook the Steak: Heat a heavy pan over high heat and add vegetable oil. Place the steak in the pan and cook for about 2 to 3 minutes per side depending on thickness. Aim for a juicy center.
Rest and Slice: Remove the steak from the pan and let it rest for 5 minutes. Slice thinly against the grain to keep it tender.
Prepare the Bread: Slice the Turkish bread lengthwise. Lightly toast the inside if you like extra texture.
Assemble the Sandwich: Spread garlic mayo on the bread. Add sliced steak, caramelized onions, tomato slices, rocket, and a light swipe of mustard. Close the sandwich gently and slice to serve.
