In a large mixing bowl toss the sweet potato slices with the oil and all the other ingredients until the slices are well coated and the garlic and chilli well-distributed.
Transfer to a lightly oiled gratin dish, spreading out the slices with your fingertips, you do not have to layer the gratin piece by piece, but try to ensure that the slices are mostly lying flat.
Pour over any cream remaining in the bowl and trickle the remaining oil over the gratin.
Bake in a preheated, fairly hot oven (180C/350F/Gas 5) for 40-50 minutes until the sweet potato is completely tender and the top is browned and crispy.
For extra crispness you can finish under the grill for 1-2 minutes.