Tip all of the potatoes into a large pot of cold water. Heavily season with salt and chuck in the stock pot, fresh garlic and onion, thyme and rosemary. Place on the hob on a high heat and bring to a boil, then cook for 10-15 minutes until tender.
Preheat the oven to 200ºc fan/220ºc/400ºf. When the potatoes are soft, remove the garlic, onion and herbs. Drain into a colander and place it back over the hot pan you cooked them in, then top with a clean tea towel and leave to steam dry for 10-15 minutes.
Place the beef fat onto a baking tray and pop into the oven for 10-15 minutes while the potatoes are steaming. Grab a wire rack and carefully lay out the potatoes in a single layer.
Mix together the rice flour, garlic granules, onion granules and a big pinch of salt. Use a sieve to shake a thin layer of the rice flour mix over the potatoes, turning them over to get all sides. Carefully place all of the potatoes into the hot beef fat and turn them over to coat.
Roast for 30 minutes, then give the tray a shake and turn the potatoes. Continue to roast for another 30 minutes until golden brown all over and super crunchy.