Beautiful and delicious, but super-fiddly
Hasselback potatoes have nothing to do with David Hasselhoff, so don't let your mind take you anywhere in that direction. (Nothing good lies that way.) They are the specialty potatoes of a restaurant in Sweden called Hasselbackens, where they are on the menu as Hasselbackspotatis ("Hasselback's potatoes").
Having made these, I can totally believe that they sprang from a super-fancy restaurant. You know… the kind that has prep staff to do all the tedious work.
The basic idea is that you slice a potato almost but not quite all the way, so that the slices are being held together by the bottom part of the potato. Then you shove stuff in between the slices to get them to fan out and be more delicious. And finally, you roast them for a while to make them crispy and delicious.
My interest in Hasselback potatoes was piqued by a Pinterest entry. It just showed a picture of the finished potatoes with the instructions to slice and bake. I might have had better results if I had practiced or researched first. Pfft! Research is for losers. (And I am clearly a loser. Now.)
Food blogs have been going nuts over Hasselback potatoes for a few years now. This makes sense, because it is the perfect combination of "pretty and photogenic" and "super fiddly and time-consuming." First you have to slice it correctly. Not all the way through! Guess how many times I sliced my potato all the way through by mistake. (Hint: 3.)
Protip: you might have better luck if you shave a slice off the back of the potato first, so that it has a flat spot to rest on. Mine rolled all around. INFURIATING.
Next, jab small splinters of stuff between the slices. I have seen people use slivers of garlic, alternate thin slices of butter with thin slices of romano cheese, matchsticks of onion, rosemary stems, and more. This gets the slices baked properly, and makes them fan out prettily.
In the final stage, you can also cover them with bread crumbs, cheese, cream, or whatever else delicious you want on there. I even found a recipe that simmers the potatoes in chicken stock and saffron, super fancy!
According to a comment I saw somewhere, the "real" Hasselback potatoes are just covered with butter and bread crumbs and baked like any other potato. Frankly, that is probably the approach I will take if I try this again.