Hasselback Potatoes with Butter, Garlic and Rosemary

Preheat a conventional oven (no fan) to 225°C and set a rack in the middle. Have ready a shallow metal baking sheet about 30 × 40 cm, a pastry brush and two wooden spoon handles to stop the knife from cutting too deeply. Choose eight similarly sized potatoes, preferably not too round; scrub them and dry thoroughly, leaving their thin skins on.

Set a potato securely on the board, lengthwise between the wooden handles, and make crosswise cuts about 2–3 mm apart. Let the knife stop against the handles, leaving about 1 cm of the base uncut. Make shallower cuts at both ends, where the handles may not stop the blade on the rounded surface. Repeat with the remaining potatoes: keep the slices even and connected, without pulling them off or forcing them apart.

Melt 40 g butter with all the olive oil over low heat, just until liquid, then remove from the heat. Arrange the potatoes on the baking sheet with the cuts facing up, leaving about 2 cm between them. Brush with roughly half the mixture, running the brush along the gaps too, and distribute all the salt and pepper over the potatoes. Reserve the other half of the mixture for the next brushing; roast for about 40 minutes, until the surface starts to brown and the cuts begin to open.

Carefully pull out the baking sheet and set it on a heatproof surface. Brush the reserved half of the butter and olive oil over the potatoes and between the opened slices; if the mixture has solidified, briefly warm it over low heat. Return the sheet to the oven for about 20 minutes, until the fans open further and their edges turn golden. Meanwhile, finely chop the garlic and rosemary leaves.

Test the thicker center of one potato with a thin knife: it should already be nearly tender. If it still offers noticeable resistance, roast the potatoes for about 5–10 minutes longer before adding the garlic. Melt the remaining 20 g butter over low heat, remove from the heat and stir in all the chopped garlic and rosemary. Brush the flavored butter between the open slices and over the tops; do not leave large clumps of garlic at the tips.

Roast for about 10–15 minutes longer at the same temperature, until the edges are browned and crisp and a thin knife passes through the thicker center without resistance. Total roasting time is usually about 70–75 minutes; firmer or larger potatoes take longer. If the tops brown faster than the centers soften, move the sheet down one rack position and roast for about 5 minutes longer. The garlic should stay lightly golden, without dark, burned clumps.

Use a wide spatula to transfer the hot potatoes to plates, allowing two whole fans for each of four servings. Serve immediately as a side dish. Do not cover them after roasting so steam does not soften the crisp edges.