This is easy but I HIGHLY reccomend useing a mandoline to slice the potatoes so you get uniform tickness. This is the reason I gave it a medium rating on ease of cooking! It makes quite a presentation at the dinner table.
Preheat oven to 375 degrees. In a small bowl, combine butter and oil. Brush bottom of a round 9-inch baking dish with some butter mixture. With a sharp knife or mandoline, slice potatoes very thinly crosswise.
Arrange potato slices vertically in dish. Wedge shallots throughout. Sprinkle with salt, brush with remaining butter mixture. Bake 1 1/4 hours. Add thyme and bake until potatoes are cooked through with a crisp top, about 35 minutes more.