Prepare your barbecue first. Make sure the coals or charcoal are white hot before starting. Season your steaks and cook them for 2-3 minutes each side until they start to brown. Remove from the heat and cover with foil to rest for 5 minutes.
Butter the bread on one side. In a single layer, cover your sandwich with cheese. Break off pieces to fill any gaps you may have all the way to the edge.
Add a thin layer of sliced onion and some fresh black pepper. Thinly slice the steak at an angle and add more cheese on top to your liking.
Prepare your grill basket. Place the top slice of your sandwich with the filling inside the cage, then add the second slice with the butter side facing up on top. Continue until your grill cage is full.
Secure the cage properly and place it over the hot coals. Make sure the cage doesn’t physically touch or rest on the coals themselves.
Leave it for 3-4 minutes with the barbecue lid closed before checking. Flip when nice and golden brown and continue on the other side. Cut into triangles and enjoy immediately while the cheese is hot and delicious.