Step 1

Position rack in upper third of oven; preheat to 450°F.

Step 2

Toss sweet 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 might shatter.) Roast, flipping once, until browned, about 25 minutes.

Step 3

Whisk broth, brown sugar, vanilla, cinnamon and nutmeg together in a measuring cup. Carefully add the mixture to the pan. Continue roasting for another 10 minutes. Top each slice with marshmallows; roast until the potatoes are very tender and the marshmallows are just melted, 3 to 5 minutes more.