Drop the sweet potatoes into a saucepan of salted boiling water and simmer for 5 minutes, then drain.
Melt half the butter with the oil in a flameproof dish and stir in the honey and ginger. Add the sweet potatoes and toss in the honey mixture. Dot with the remaining butter, then season with salt and pepper.
Cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 40 minutes, or until the potatoes are tender. Brush with the glaze in the pan and turn occasionally during cooking.