Put the garlic cloves and olive oil in a very small saucepan, making sure the cloves are submerged. Cook gently over low heat for about 12 minutes, until the garlic softens and begins to take on colour. Remove the pan from the heat and set aside for the oil to cool completely.

When the oil has cooled, transfer the garlic to a bowl (leave the oil in the pan), and mash roughly with a fork.

Put the flour, yeast, pecorino, salt and pepper in a medium bowl. Stir together with a hand whisk to combine, then add the mashed garlic and the water. Mix well with a flexible spatula and scrape the dough from the side of the bowl, folding it into the centre until a sticky ball forms.

Drizzle 15ml of the reserved garlic oil on top of the dough ball and use your fingers to gently spread the oil all over the surface of the dough. Cover the bowl with a large plate or with plastic wrap and place immediately into the fridge to prove slowly overnight (and for up to 2 days).

The next day, remove the dough from the fridge, but keep it in the bowl. Pour 30ml of the reserved garlic oil into the base of a 30cm round tin (a rectangular baking tray approximately 30×20cm would also work well). Using your dominant hand, smear the oil over the base of the tin.

Now, with the dough still in the bowl, use your oiled hand to pull the sides of the dough out and over to the centre. Rotate the bowl and repeat a few times to firm up the dough, then transfer it to the oiled tin. Turn the dough to coat it in the oil, then leave it to rest and relax on the kitchen bench, uncovered, until it is soft and airy with a puffy surface. This will take 1–2 hours.

When the dough is nearly ready, preheat the oven to 220˚C fan-forced. Cut the potatoes (unpeeled) into 2–3mm slices.

Drizzle 2 more tbsp of the garlic oil on top of the dough, then press your fingertips and thumbs into the risen dough a few times to leave deep, oily dimples all over. Now layer the potatoes on top, slightly shingled. Tear off small sprigs of the rosemary and tuck them into and around the potatoes. Drizzle the top with the remaining garlic oil and sprinkle with some flaky sea salt.

Bake for about 30 minutes, until the potato topping and the underside of the focaccia are golden brown. Place the tin on a wire rack to cool for a few minutes before lifting the focaccia out of the tin to cool further on the rack.