Preheat and Prep: First, place a large, heavy-bottomed baking tray into the oven and preheat it to 220°C (200°C fan). Getting the tray scorching hot is vital for a crispy base.

Boil the Potatoes: Place the potatoes in a large pot and cover them with cold, generously salted water. Bring to a boil and cook for 15-20 minutes, or until they are completely tender and a sharp knife can be inserted with no resistance.

Drain and Steam Dry: Drain the potatoes well in a colander. Then, return the empty pot to the low heat for a moment to evaporate any remaining water. Tip the potatoes back into the dry pot, put the lid on, and let them sit off the heat for 5 minutes. This step allows the surface of the potatoes to become dry and fluffy, which is the secret to a crunchy finish.

Prepare for Roasting: Carefully remove the hot baking tray from the oven. Pour the 4 tablespoons of olive oil onto the tray – it should shimmer immediately. Add the steam-dried potatoes to the hot oil, along with the unpeeled garlic cloves and rosemary sprigs. Gently toss everything together to ensure each potato is well-coated in oil.

Smash the Potatoes: Using the base of a sturdy glass or a potato masher, gently press down on each potato until it's smashed to about 1.5-2cm thick. You want it to split and flatten, but not fall apart completely. What works best for me is a firm but gentle, single press.

Season and Roast: Sprinkle the smashed potatoes generously with the flaky sea salt and freshly ground black pepper. Place the tray back into the hot oven.

Roast to Perfection: Roast for 30-35 minutes, turning them over halfway through, until they are deep golden brown, crisp, and the edges are wonderfully craggy. Keep an eye on them in the last 10 minutes to ensure they don't burn.

Serve Immediately: For the best texture, serve these crispy potatoes straight from the oven. The soft, sweet roasted garlic can be squeezed out of its skin and mashed into the potatoes if you wish.