First, cut the potato into 8 wedges.
Place the wedges into a pan of boiling water and simmer for 2-3 minutes. This will soften them up so that the insides will be fluffy. You can skip this step, but your wedges won't be quite as soft in the middle.
Leave to cool and dry for at least 10 minutes. In the meanwhile, pre-heat the oven to 200C / 390F.
Lay the wedges flat on a lightly oiled baking tray, and brush the tops with additional oil. Sprinkle salt over the top, and then place in the oven.
While the wedges are cooking, make the truffle mayonnaise. Using the emulsifying attachment on your food processor or hand mixer, whisk the egg yolk for a few seconds before adding the mustard, lemon juice and salt. Whisk until just combined. Add the oil in a very slow stream, starting with the truffle oil. After you have added 1/4 cup of oil, open the lid and check out the texture. If it's thick enough, you can stop there. If you would prefer it a little thicker, add more oil, a little at a time, until you have it at the thickness you enjoy. For me, that was 1/4 cup + about 2 tablespoons.
After the wedges have been in the oven for around 25 minutes, they should be fairly browned on the top but not totally cooked. Once they're at that stage, take out from the oven, give the pan a shake to loosen them up, and then flip them all over individually. I know - it's a little tedious! Now sprinkle with the garlic and sage and put back into the oven for another 15 to 20 minutes.
Once the garlic and sage are crisp and fragrant, and the wedges have browned a little on top again, they are good to go!
Place them on a serving platter and sprinkle all the sage and garlic bits from the pan on top.