Preheat the oven to 220 C or 425 F. A fan oven works best, but a conventional oven is fine.
Peel the sweet potatoes if you prefer, then slice them into fries.
Place the fries in a bowl and toss lightly with olive oil. Use only enough to coat them without pooling.
Spread the fries out on a large baking sheet. Do not overcrowd the tray. Space helps them brown and reduces steam.
Add a few sprigs of thyme if you like.
Roast for 30 to 35 minutes until golden. You can flip them once, but they usually colour well without turning.
If the fries feel a little soft after roasting, place them on a wire cooling rack set over a baking tray and return them to the oven on a very low heat for 10 to 20 minutes to dry out.