Preheat oven to 450°F.

Pour the salt into a baking dish. Nestle the potatoes into the salt, and arrange the rosemary, thyme and garlic cloves around them in the salt. Cover tightly with foil and roast for one hour. If using large potatoes, test for doneness with a paring knife - the tip of the knife should easily pierce the potato.

Remove the foil and brush the all sides of the potatoes with olive oil. Remove the garlic cloves and set aside. Discard the rosemary and thyme springs. Return the pan to the oven and bake, uncovered, for another 15 minutes, to crisp the skin. Meanwhile, squeeze the roasted garlic cloves into the butter and mash well.

Brush any caked salt from the potatoes. Slice open and garnish with garlic butter, sour cream and a sprinkle of herbs de Provence.