Preheat the oven to 220 °C.
Wash sweet potatoes very well then dry with kitchen paper. Do not peel them!
For each sweet potato, cut a very thin slice lengthwise from the bottom so they can stand without rolling.
Slice into each sweet potato crosswise at 4 mm intervals. Be careful! The slices should stay together at the bottom. I used wooden spoons lengthwise on opposite sides of the potatoes to avoid the cutting.
Make the hasselback with bacon. Chop the bacon slices to 5 cm pieces. Push a piece of bacon to each intervals of sweet potato. Spread with 1 tbsp butter. Season with salt and pepper. Do not season it with salt if the bacon is salted.
Make the cheesy hasselback. Spread the top of the sweet potato with 1 tbsp butter. Sprinkle with chopped rosemary. Season with salt and pepper. The cheese is added after the prebaking.
Place sweet potatoes to baking tray. Cover with foil and bake for 40 minutes.
Uncover potatoes. Finish the cheesy hasselback. Use a butter knife to open the layers a bit and sprinkle the grated cheese between them. Drizzle the top of the bacon hasselback with truffle oil.
Put sweet potato hasselbacks back to the oven and roast them for about 15 minutes, until they are tender, and the edges are crisp. Enjoy!