1. Preheat oven to 190C (375F).
2. In a small bowl combine melted butter and olive oil.
3. Peel the potatoes. Then with a sharp knife or mandoline, slice potatoes very thinly crosswise.
4. Brush bottom of a large round baking dish and arrange potato slices vertically. brush the potatoes with 2/3 of the butter mixture. Sprinkle with salt and pepper.
5. Bake for 60-75 minutes. Add thyme springs and brush with the remaining butter. Bake until potatoes are cooked through and crispy on top, about 30 minutes more.