Hasselback Potatoes

Place a rack in lower third of oven and preheat to 425°. Place a potato from 3 lb. Yukon Gold potatoes (8–12, depending on their size), scrubbed, on a cutting board so a long side is facing you and arrange a chopstick along each long side. Using a long, thin knife, cut potato crosswise into ⅛"-thick slices, stopping at chopsticks (you don’t want to cut all the way through). Transfer to a large bowl and repeat with remaining potatoes.

Drizzle ⅓ cup extra-virgin olive oil over potatoes, then sprinkle 1 Tbsp. plus 1 tsp. Diamond Crystal or 2¼ tsp. Morton kosher salt, and 1 tsp. freshly ground pepper over. Using your hands, massage oil and seasonings into slits in potatoes.

Transfer potatoes to a parchment-lined rimmed baking sheet and arrange in a single layer, slit side up, spacing at least 1½" apart. Tightly cover with foil and bake potatoes until just barely tender, 30–35 minutes.

Meanwhile, mix together 5 garlic cloves, finely chopped, ½ cup (1 stick) unsalted butter, melted, remaining 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and remaining ½ tsp. freshly ground pepper in a small bowl.

Uncover potatoes and brush about half of butter mixture evenly over tops, taking care to work some into slits. Continue baking potatoes, uncovered, until tops are golden brown and crisp to the touch, 45–55 minutes longer.

Using a thin spatula, transfer potatoes to a platter. Mix 2 Tbsp. finely chopped parsley into remaining butter mixture and brush over potatoes. Scatter remaining 1 Tbsp. finely chopped parsley on top, then sprinkle finely grated Parmesan and flaky sea salt over if desired.