Preheat a cast-iron or nonstick skillet on medium-low for 2–3 min.
Spread a thin, edge-to-edge coat of butter or mayo on the outside of each bread slice.
Build each sandwich: half shredded cheese on the bottom, slices on top for stretch. Keep fillings thin if using.
Cook 3–5 min on the first side. Press lightly for contact and rotate the pan for even browning.
Flip once. Cook 2–4 min more. If cheese lags, cover 30–60 sec, then uncover to re-crisp.
Rest on a wire rack 1–2 min. Slice on the bias. Serve hot and crisp.