Peel the plantains and cut them crossways into slices 2.5cm thick. Lay each plantain on a large sheet of foil, keeping the slices together so they stay in their original shape.
Smear each plaintain with the butter (or dot it over if it’s hard) and sprinkle with the sugar. Make a gondola shape round each plantain with the foil, then pour over the orange juice and grind salt and pepper on top. Wrap the foil completely round the plantains so they’re securely sealed inside. (You can prepare to this stage up to 24 hours ahead.)
Put the foil parcels on the barbecue rack and cook for 30 minutes until the plantains are softened (to check if they’re ready, open a parcel and test with the point of a knife).