Halving the yams before roasting greatly reduces the cooking time and gives them a nice golden crust.
Preheat oven to 400 degrees. Halve yams, and place, cut side up, in a shallow baking dish just large enough to hold them in a single layer. Drizzle with oil, and sprinkle with thyme. Season with salt and pepper. Bake until golden brown and very tender, 35 to 45 minutes.