Instructions:
- Wash potatoes thoroughly. Cut into uniform 1.5-inch (4cm) chunks, leaving the skins on.
- Place potatoes in a large saucepan. Cover with cold water and add 1 Tbsp coarse salt. Bring to a rolling boil over high heat.
- Boil for 10–15 minutes. Test for doneness: a sharp knife should slide in with very little resistance, but the potato should not fall apart.
- Drain the potatoes immediately into a colander and let them steam dry for 5 minutes. Gently shake the colander vigorously for about 30 seconds until the edges of the potatoes look ragged and slightly mashed (this creates maximum surface area for crisping!).
- Preheat the oven to 425°F (220°C). Place the olive oil/fat and butter mixture onto your metal baking sheet and place the tray in the oven while it preheats until the fat is smoking hot.
- Carefully remove the hot tray. Gently place the roughed-up potatoes onto the sizzling fat, ensuring they are in a single layer. Toss gently to coat.
- Scatter the rosemary sprigs and smashed garlic cloves amongst the potatoes.
- Return the tray to the oven and roast for 20 minutes.
- Remove the tray, turn the potatoes using a spatula to expose all sides. Return to the oven and roast for another 15–20 minutes, or until deeply golden brown and exceptionally crispy.
- Remove from the oven. Transfer to a serving bowl. Immediately sprinkle generously with flaky sea salt and freshly cracked black pepper. Serve piping hot.