Preheat oven to 450 F and place a large, heavy skillet or roasting pan in the oven to heat.
Place the potatoes in a large saucepan and cover with cold salted water. Bring to a boil over high heat. Once boiling, lower the heat and simmer for 5 minutes.
Drain the potatoes well and place them on paper towels. Put the flour on a plate. When the potatoes are dry, roll them in the flour to coat.
Remove the hot skillet from the oven and add the oil and potatoes. Roast, shaking the pan occasionally until the potatoes are golden brown all over, about 30 minutes.
Remove from the oven and add the garlic, sage and butter. Return to the oven for one minute.
Season generously with salt and pepper and serve.