1. Preheat oven to 475°. Rinse fennel thoroughly. Trim and discard root end of fennel bulb. Trim stalks from bulb, reserving fronds for another use. Thinly slice bulb.
2. Add oil to a 9-inch cast-iron skillet. Arrange half of potato slices in skillet. Layer with fennel slices and remaining potatoes. Sprinkle with thyme, salt, and pepper. Cover with aluminum foil.
3. Bake at 475° for 35 minutes. Uncover and bake 15 more minutes or until vegetables are browned. Serve hot or at room temperature. If desired, transfer to serving bowl.