Method
Drain the mozzarella
Slice the mozzarella and lay it on kitchen paper for 10 minutes. Wet mozzarella in a pressed sandwich makes the bread soggy and steams rather than melts. This step matters.
Spread the 'nduja
Slice the roll and spread 'nduja generously on both cut sides — it will melt into the bread as it presses. Don't be shy; its heat and fat are the whole point of this sandwich.
Layer and close
Lay the mozzarella slices over the 'nduja on the bottom half. Add the basil leaves. If using honey, drizzle it very sparingly over the mozzarella — it cuts the heat of the 'nduja in an interesting way. Close the sandwich firmly.
Press hard and hot
Heat a panino press or a heavy cast iron pan to medium-high. Brush lightly with olive oil. Press the sandwich for 3–4 minutes per side, applying firm pressure, until the bread is deeply golden and blistered and the mozzarella has completely melted. Cut in half and eat immediately.
On 'nduja: Quality varies enormously. A good 'nduja should be spreadable at room temperature, deeply red, fatty, and aggressively spiced. If yours is too firm to spread easily, warm it briefly in a pan first. Avoid 'nduja that smells more of vinegar than pork.