Boil the potatoes in a large pan of water, until they are fork-tender (i.e. soft but not so soft they are falling apart).
Once they are cooked, drain the potatoes well, return to the pan, and place a lid on. Holding the lid in place, shake the pan a few times in order to slightly break up the surface of the potatoes.
Gently stir in the mayo and smashed garlic cloves, then turn out into a cast iron skillet or baking tray.
Sprinkle with rosemary, sea salt, and black pepper, then roast in the centre of the oven for around 40-45 minutes, until the outside of the potatoes has crisped up to your liking.
Serve immediately.
These mayo-roast potatoes will keep for 2-3 days in an airtight container in the 'fridge. Although they can be frozen, I don't recommend it, as it alters the texture somewhat.