Peel the sweet potatoes and slice about 1-1/4 inches thick. Don't slice too thin or they won't stay in tact.

Add 1/2 teaspoon salt to water in a saucepan and bring to a boil. Add potato slices and simmer for about 15-20 minutes, or until somewhat softened. Remove carefully to a wire rack and allow to cool for about 10 minutes.

Line a large, heavy sheet pan with foil. Carefully transfer slices to pan. Using a flat-bottomed cup or back of a spatula, slightly mash each potato slice, taking care that they stay intact. Drizzle each slice with a mixture of the butter and oil, using half of the mixture. Brush with a pastry brush to coat.

Blend the seasoning ingredients (sugar and all the spices) together really well with a fork, making sure to get the sugar completely incorporated.

Sprinkle the slices evenly with a half the seasoning mixture.

Gently turn over each slice, drizzle with the remaining butter/oil mixture and sprinkle with the remaining seasonings.

Roast at 375° for 20 minutes, carefully flip each slice and return to oven for about 10 minutes more.

Serve hot.