To make the butter, mix together the butter, mustard and honey and season well.
Open the outer leaves of the cabbage to expose the main ball, spread out the leaves and make a few big cuts into the heart. Smear half the butter mix over the middle, draw the outer leaves back up and secure with string (this will stop the leaves falling off).
Set the cabbage in the coals and roast for 30 minutes, until the outer leaves are charred and an inserted skewer doesn’t offer much resistance. Remove the cabbage and cut off the string, then remove the outer leaves to expose the cooked middle. Cut into wedges and baste with the remaining butter in the tray while warm. Drizzle with olive oil.