Heat the oven to 200°C. Bring a large pan of salted water to the boil. Boil the potatoes until almost cooked but still firm. Drain and allow to cool a bit. Heat the goose fat and olive oil in a roasting tin, while you cut the potatoes in half lengthwise.
Put them into the hot fat with the whole cloves of garlic, the rosemary, bay leaves and thyme. Jumble the potatoes round in the fat until they are well-coated and roast until lightly browned, about 25–30 minutes, turning them occasionally.
Take them out of the oven and pour off some of the fat. You can set them aside at this point, ready to finish off when you want them.
Well before you want to serve the potatoes, turn up the oven to 250°C. Remove the bay leaves and rosemary and squeeze the garlic pulo out of its skins. Jumble the potatoes round to roughen and break them a bit, scatter with half the parsley and and roast until crunchy, turning them over once or twice. This will take about 15 minutes. Add back the garlic pulp and stir it in 5 minutes before the end of the cooking Do not let the garlic burn, this is crucial as it tastes bitter when over-browned. Scatter on the sliced, raw garlic and the rest of the parsley at the last moment and stir them in.