Preheat oven to 425 degrees. Cut potatoes lengthwise into thick slices, then into 1/2-inch by-1/2-inch-thick strips. Place in a bowl of cold water until ready to use. Drain, and dry well on paper towels.
Toss potatoes with oil and spread out on a dark roasting pan or baking sheet. Scatter thyme over potatoes. Bake, turning occasionally, until potatoes are brown and crisp on all sides, 35 to 40 minutes.
Remove from oven, discard thyme stems (the leaves will have fallen off), sprinkle with salt and pepper, and serve immediately.