Step 1

Position rack in upper third of oven; preheat to 500°F.

Step 2

Toss potatoes, butter, oil, salt and pepper in a large bowl. Arrange in a single layer in a 9-by-13-inch metal baking pan. (Don't use a glass dish, which could shatter.) Roast, flipping once, until browned, about 30 minutes.

Step 3

Stir broth and vinegar together in a glass measuring cup. Carefully add to the pan and continue roasting until most of the broth is absorbed and the potatoes are very tender, about 15 minutes more. Sprinkle with flaky salt and scallions. Serve hot.