Step 1

Preheat oven to 400 degrees with the rack in the middle. Add the oil to a rimmed sheet pan (13-by-18-inches) and place it in the oven.

Step 2

Meanwhile, bring a large pot of water to a boil. Add the potatoes and boil until softened, 4 to 5 minutes. Drain the potatoes in a colander and sprinkle with the flour. Shake the colander to distribute the flour evenly.

Step 3

Carefully remove the pan with the hot oil from the oven. Spread the potatoes out in a single layer in the hot pan. The potatoes should sizzle upon making contact with the hot oil. Return the baking sheet to the oven and roast the potatoes for about 1 hour, until golden brown. Every 15 to 20 minutes, flip the potato pieces, ensuring that all sides brown evenly. About 45 minutes into cooking, sprinkle the potatoes with the rosemary and/or the garlic, if using. Transfer to a cooling rack and generously season with the salt and serve.