Peel and cut potatoes into large cubes. Add the potatoes into a large pot and fill with cold water. Place the pot on high heat and bring the water to a boil. Once the water has boiled, cook the potatoes for an additional 6 minutes until they are parboiled.
Drain the parboiled potatoes into a colander; season with salt and pepper, and toss in flour so that all the potato cubes are lightly coated and fluffed. This is how the potatoes get their crispy texture.
Preheat oven at 220°C. Pour oil into a baking sheet and place in the oven to heat until it is very hot. (The baking pan needs to be large enough so that all the potatoes are in a single layer.) Remove the baking sheet from the oven, and add chopped garlic and rosemary to the heated oil. Mix together for the flavours to infuse. You will hear a wonderful sizzle at this point. Add in the parboiled potatoes and toss the ingredients together until the potatoes are coated, and evenly spread in the baking sheet.
Lower the oven temperature to 200°C and roast the potatoes in the oven for about 45–50 minutes, until the potatoes are golden-brown in colour. Half-way through baking time, toss the potatoes with a spatula to ensure all sides are evenly brown. Remove from the oven and serve hot.