Cut each grapefruit in half through the middle (horizontally, not pole to pole). Trim a small slice from the bottom of each half if necessary to prevent them from rolling.
Place the halves cut-side down on kitchen paper for 1-2 minutes to absorb excess moisture. This helps the sugar caramelise properly.
Transfer the grapefruit halves to a baking tray, cut-side up.
Grind a few twists of black pepper over each grapefruit half.
Sprinkle 1 tablespoon of sugar evenly over each half. If your grapefruits are small, use slightly less sugar.
Using a kitchen blowtorch, hover the flame over the sugar and move it around in circular motions, ensuring all the sugar melts and caramelises. This takes 1-2 minutes per half. Keep the flame moving to prevent scorching.
The sugar will bubble, turn golden, and harden into a crisp crust. Serve immediately whilst the caramelised topping is still crisp.