Preheat oven to 410F/210C.
Place potatoes in a pot of cold water and bring to boil with a hefty pinch of salt. Boil until the potatoes are fork tender, but not so much they begin breaking apart. You want the potatoes fairly soft (slightly past parboiled). Softer spuds = rougher edges = extra crispiness.
Drain in a colander (do not rinse) and give them a shake to rough the edges. Allow them to steam dry for 5-10mins. The more they steam = the more moisture is leaving the potatoes = the fluffier they'll be. During this time, spread your goose fat in a suitably sized baking dish and place in the oven. The fat should comfortably cover the base of the dish.
Mix 1 heaped tsp (not tbsp!) flour with a hefty pinch of salt and some pepper (to taste). Then sprinkle over the potatoes, shaking a little more to ensure they’re evenly coated.
When your goose fat is smoking hot (literally) very carefully add your potatoes to the pan, ensuring they’re evenly spaced out. Add garlic and rosemary and stir to coat everything in the fat. Work quickly so the fat stays hot, just be careful of the fat spitting out the pan.
Place in the oven for a total of 50-60mins or until golden and crispy on the outside. Take out and flip/re-coat in fat once/twice throughout to ensure an even coverage.
Serve with an extra sprinkle of salt to drawer out the final bit of moisture, just for extra crispiness.