Preheat the oven (150C fan/160C conventional) and line 2 large roasting trays or cookie sheets with baking paper.
Remove the leaves from the stems and tear into pieces. Rinse thoroughly and then either use a salad spinner to dry or pat dry with kitchen towel. It’s really important that the leaves are dry otherwise they will steam rather than roast in the oven.
Massage the oil into the leaves so they are well coated. Spread over the trays, spaced apart.
It is best to cook these in batches, one tray at a time. Roast for 10 minutes, rotate the tray and roast for a further 10 or until the leaves are shrunken and crisp. Keep an eye on them as they can scorch! Repeat with the second tray.