Double Potato Gratin

3 lb (1.5 kg) Yukon Gold or baking potatoes (about 6 large)

1 lb (500 g) Sweet Potatoes (about 2)

1 tbsp butter, divided

1/4 cup olive oil

2 tsp salt

1 tsp pepper

1 tbsp chopped fresh rosemary or thyme (or 1/2 tsp dried)

1. Peel potatoes and sweet potatoes.  Cut them into slices 1/4 inch thick.
2. Place a piece of parchment paper at the bottom of a 9 inch baking dish and cover with half the butter.  Arrange a layer of regular potatoes over bottom of dish.  Sprinkle potatoes with a little olive oil, salt, pepper and rosemary.  Continue to layer and sprinkle potatoes until you have used half the potatoes.
3. Arrange sweet potato slices on top, sprinkling with oil, salt, pepper and rosemary after each layer.
4. Continue to arrange remaining potatoes in layers on top of sweet potatoes, sprinkling after each layer.
5. Butter another piece of parchment paper with remaining butter.  Place paper, butter-side down, on top of potatoes.  Weigh down paper with a heavy oven proof dish or foil covered bricks.
6. Bake in a preheated 425F oven for 1 hour, or until potatoes are tender.  Remove weights and the top paper.  Let potatoes sit for 5 minutes.  
7. Invert dish onto a serving platter and remove remaining piece of parchment paper.  Serve with the best looking side up.