Place a rack in lowest position in oven and preheat to 425°. Halve 2 lb. Yukon Gold potatoes crosswise.
Arrange potatoes, cut side down, on a rimmed baking sheet, spacing evenly apart. Pour in 1½–2 cups water (it needs to cover baking sheet in a thin layer).
Cover baking sheet with a double layer of foil, crimping sides very tightly to seal well. Doing this will trap in steam and prevent water from dribbling out if you tip the sheet.
Bake potatoes until a fork or skewer slides easily through potatoes, 25–30 minutes. The foil will be puffed, but poke through and feel around for a potato to test. Try testing a couple to gauge doneness.
Let potatoes cool about 5 minutes. Crank up oven temperature to 500° (or as high as your oven goes). Use tongs to lift up foil at 1 corner, peeling it back only about 1". Tip open corner into the sink and pour off water.
Remove foil and drizzle ¼ cup oil over potatoes; season generously with salt and pepper. Let potatoes cool to the point you can handle them, then toss with your hands to coat completely. Arrange cut side down again.
Roast potatoes until tops are golden, bottoms are deeply browned and crisp, and oil around edges is sizzling and, 20–25 minutes.
Let potatoes cool slightly, then use a spatula to unstick them from baking sheet.