Preheat and Prep: Preheat your oven to 220°C (200°C fan). Pour the 4 tablespoons of olive oil onto a large, sturdy baking tray and place it in the oven to heat up. Getting the oil hot is essential for an initial sizzle.

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

Drain and Steam-Dry: Drain the potatoes thoroughly in a colander. Let them sit in the colander for 3-5 minutes to steam-dry. I used to struggle with this dish until I discovered this technique; shaking them gently helps release more steam and roughs up the edges a bit, creating more surface area for crisping.

Smash the Potatoes: Carefully remove the hot tray from the oven. Tip the dry potatoes onto the tray into the hot oil – they should sizzle immediately. Spread them out in a single layer. Using the base of a glass or a potato masher, gently but firmly press down on each potato until it's about 1.5cm (just over half an inch) thick. Don't press so hard that they fall apart completely.

Season Generously: Drizzle a little more olive oil over the smashed potatoes. If using, drizzle over the melted butter. Scatter the unpeeled garlic cloves and sprigs of rosemary across the tray. Season generously with the flaky sea salt and black pepper.

Roast to Perfection: Place the tray back into the hot oven and roast for 25-30 minutes. By this point, the bottoms should be golden and crispy.

Flip and Finish: Remove the tray from the oven. Using a metal spatula, carefully flip each potato. What works best for me is to work methodically from one corner to the other. Return to the oven and roast for another 20-25 minutes, or until the potatoes are deeply golden, crisp all over, and the edges are beautifully browned.

Serve Immediately: Remove the crispy smashed potatoes from the oven, discard the rosemary stalks if you wish (or leave for presentation), and squeeze the soft, roasted garlic from their skins over the potatoes. Give everything a final toss and serve straight away while they're at their crispy best.