Rosemaried Sweet Potatoes

Ingredients
Large knob of butter
4 Sprigs of rosemary
2 x 425g (15 oz) sweet potatoes
Salt and freshly ground black pepper

Method
Lightly butter four large squares of foil and place a sprig of rosemary in the centre of each.
Dice the sweet potatoes and divide them between the squares. Season generously and then dot with a little more butter. Fold over the foil and scrunch up the edges to enclose the potatoes in little parcels.
Cook on a rack on a barbecue over a medium coals for about 20-30 minutes (or if the great british summer takes over, the oven at 180 C for the same amount of time) shaking the parcels occasionally until the sweet potatoes are tender, and a little charred.