Instructions: Peel all potatoes. Using a mandoline or a sharp knife, slice potatoes thinly, keeping sliced white and and sweet potatoes in separate piles.

In a 10-inch cast-iron skillet, heat oil over medium heat. Add onion and cook until tender. Transfer onion to a bowl. In the same skillet, arrange white potatoes, overlapping them slightly. Sprinkle with onion; dot with 2 tablesppons butter. Sprinkle with 3/4 teaspoon salt and 1/4 teaspoon pepper.

Arrange sweet-potato slices over white potatoes. Dot with remaining butter and sprinkle this layer with salt and pepper as well.

Cook layered potato mixture over medium heat for 15 minutes or until bottom layer of potatoes is lightly browned. Meanwhile, preheat oven to 450 F.

Place skillet on bottom rack in oven and bake 25 minutes or until potatoes are tender when pierced with a kife, pressing potatoes down with pancake turner occasionally. To serve, loosen edge with pancake turner; carefully invert potatoes onto warm platter. Garnish with parsley sprigs and cut into wedges.

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