1. Preheat the oven to 200 degrees.
2. Cut the sweet potato into 4-5mm thick slices, with the skin on.
3. Place the sweet potato along with the sage, garlic in a bowl and combine, seasoning well.
4. Grease a 30cm x 20cm baking dish, then arrange the sweet potatoes in parallel lines, standing to attention, in the dish. You should get three lines running vertically.
5. Make sure you transfer all the sage and garlic into the dish, then cover tightly with foil and bake for 45 minutes.
6. Remove the dish from the oven, take off the foil then pour over the double cream.
7. Bake uncovered for a further 25 minutes or until the cream has thickened and sweet potato is soft (test using a sharp knife).
8. Serve immediately, with a little more sage as a garnish as well as a sprinkle of sea salt.