Line bottom of an 8- 10 cup baking dish with parchment paper& coat with half the butter.
Peel both types of potatoes, cut into 1/4 inch thick slices& arrange the half of the regular potatoes in layers in the prepared dish; sprinkle each layer with some of the olive oil& some of the seasonings.
Arrange all of the sweet potatoes on top; sprinkle with some of the olive oil& seasonings.
Continue layering the remaining regular potatoes, sprinkling each layer as before.
Coat another piece of parchment paper with the remaining butter& place, butter side down, on top of the potatoes.
Weigh this down with a heavy, oven-proof dish (or foil covered bricks) and bake for an hour or until potatoes are tender.
Remove weights& paper; let potatoes rest for 5 minutes before inverting onto a serving dish& removing other piece of parchment.