Step 1

Preheat oven to 425°. Place potatoes in a large pot and pour in water to cover by 2". Season water generously with salt and bring to a simmer over medium-high heat. Reduce heat and simmer gently until potatoes are tender on the outside but still very firm in the center, 8–10 minutes.

Step 2

Pour off all of the water in pot, holding potatoes back (make sure there's no liquid left!). Let potatoes sit, uncovered, for a few minutes to steam off any excess liquid they've absorbed. Toss potatoes just enough to rough up their outsides and give them a floury starchy coating (do not toss so vigorously that they fall apart); season with salt.

Step 3

Meanwhile, combine both oils in a large roasting pan and heat in oven 10 minutes.

Step 4

Carefully remove pan from oven; add potatoes, turning each one to coat and moisten exterior. Make sure they're in a single layer and leave a bit of space between them. Return pan to oven and roast potatoes, turning every 10 minutes, for 30 minutes (if they aren't browning after 15 minutes, increase oven temperature to 450°).

Step 5

Toss in garlic and rosemary and cook until potatoes are deep golden brown on most of their sides, 8–10 minutes. Using a slotted spoon, transfer potatoes to a platter and serve with roasted garlic and rosemary alongside.