1. Freeze the Steak for Easy Slicing
Trim off the strip of fat from the steak and discard. Cut each steak in half, place on a tray, and freeze for about 40 minutes — this firms up the meat for easier slicing.

2. Cook the Onion & Peppers
Heat sunflower oil in a heavy pan over medium heat. Add the sliced onion and both peppers with a pinch of salt. Cook slowly for about 20 minutes, stirring occasionally, until softened and golden. Add the white wine vinegar and cook for another 5 minutes until sticky and slightly tangy. Season to taste.

3. Prepare the Steak
Remove the steak from the freezer and, using a sharp knife, slice it as thinly as possible. Divide into four portions.

4. Cook the Sandwich Filling
Heat a large skillet or griddle over high heat until very hot. Place one portion of sliced steak in the pan in a pile roughly the size of your rolls. Fry for about 3 minutes, allowing some pieces to stay slightly pink.
Top the steak pile with a quarter of the onion-pepper mix, two cheese slices, and a sprinkle of mozzarella. Let cook undisturbed for 5–10 minutes over medium heat until the edges are browned and the cheese is fully melted.

5. Assemble the Sandwiches
Split open a sub roll and scoop in the cheesy steak mixture. Repeat for the remaining portions.
Serve hot, with mustard and ketchup on the side or drizzled inside.