Peel the sweet potatoes, cut in half lengthwise and cut into thick slices. Wash thyme and shake dry. Pluck the leaves from the stems. Mix oil, salt, pepper and thyme leaves. Mix the potato slices with the seasoning oil. Pour into an ovenproof dish. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes.
Take the sweet potatoes out of the oven. Add the tomatoes. Crumble feta over it. Sprinkle with pine nuts. Bake in a hot oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for another 10-15 minutes. Take the casserole out of the oven. Sprinkle with coarse pepper and serve.