1. Scrub sweet potatoes; trim and discard any spots where skin is discolored or broken. Cut sweet potatoes in half lengthwise.
2. Pour 1/4 cup olive oil into each of two 9- by 13-inch glass baking dishes and tilt to coat pan. Sprinkle all over with salt and pepper.
3. Lay an herb sprig flat on the cut side of each sweet potato half, then lay half, cut side down, in oil (halves can touch).
4. Bake in a 425° oven until potatoes are soft when pressed on top and cut sides are golden brown (carefully lift up pan to check), about 1 hour. Using a metal spatula, carefully transfer sweet potato halves to a platter. Discard remaining oil in baking pans.