Preheat the oven to 200ºC. Lay two wooden spoons parallel across a chopping board. One at a time, place each potato between the wooden spoon handles and slice about two-thirds deep at 3mm intervals.
Place the potatoes in a large roasting tray with the cut sides facing up. Top each with half a teaspoon of duck fat, pressing it into the grooves. Sprinkle with about a third of the rosemary and season with salt and pepper. Squash the unpeeled garlic cloves under a knife and place them into the tray with the potatoes.
Roast in the oven for 1 hour and 15 minutes, or until the potatoes turn a golden colour.
Meanwhile roughly tear the bread into a baking tin and toast in the oven for 10 minutes. Remove and allow to cool. Place into a small food processor with another third of the rosemary, half the blue cheese and a good crack of salt and pepper. Pulse for 20 seconds until the bread is a rough crumb.
When the potatoes are ready, remove from the oven and top with the breadcrumbs. Crumble the remaining cheese over the top and sprinkle with the remaining third of the rosemary. Add a little olive oil to each potato and return to the oven for 15 minutes.
Transfer to a warmed serving bowl and serve immediately.