Instructions:
- Peel potatoes (optional, skin on recommended for texture) and cut into roughly 1.5-inch (4cm) uniform pieces.
- Place cut potatoes in a cold, heavy-bottomed saucepan. Cover with cold water and add 2 tablespoons of coarse salt. Bring to a vigorous boil and cook until the edges are just tender when pierced with a knife (about 10–12 minutes). Do not overcook.
- Drain the potatoes thoroughly in a colander. Return them to the empty hot pan. Place the lid on the pan and aggressively shake the potatoes for about 30 seconds to scuff up the edges, creating the starchy 'fluff' needed for crisping.
- Combine butter and oil in a large, sturdy roasting pan. Place the pan in the oven and preheat to 425°F (220°C). Heat until the fat is shimmering and just starting to smoke lightly.
- Carefully remove the hot pan. Gently place the scuffed potatoes into the shimmering fat, ensuring they are in a single layer without overcrowding. Add the smashed garlic and rosemary sprigs around the potatoes.
- Roast for 25 minutes without turning. The bottoms should become deep golden brown.
- Carefully turn the potatoes using a spatula, ensuring the browned side faces up. Return to the oven and roast for another 25–30 minutes, turning once halfway through, until deeply golden brown and crisp all over.
- Remove from the oven. Use a slotted spoon to transfer potatoes to a serving dish. Season immediately with flaky sea salt and freshly cracked black pepper. Discard the rosemary sprigs.