
Peel all potatoes. Using a sharp knife or slicer, thinly slice potatoes, keeping sweet potatoes separate.
In a 10” oven-proof skillet, heat oil over medium heat and cook onions until tender. Transfer onions to a bowl and remove skillet from heat.
In the same skillet, arrange white potatoes in a circular pattern, overlapping slightly. Sprinkle with onions and dot with 2 tablespoons of the margarine. Season with 3/4 teaspoon salt and 1/4 teaspoon pepper.
Arrange sweet potato slices in a circular pattern, overlapping slightly, over white potatoes. Dot with remaining margarine and season with remaining salt and pepper.
Cook potato mixture over medium heat for 15 Minutes until bottom layer is lightly browned.
Preheat oven to 450 degrees. Place skillet on bottom rack and bake 30 Minutes or until potatoes are tender. During baking, occasionally press potatoes down with a spatula or turner. Remove from oven and let sit 5 minutes. Loosen edges and invert potatoes onto a platter. Cut in wedges to serve.
You can also bake this dish in an oven to table deep pie dish. Preheat oven to 400 degrees. Assemble as instructed above. Bake for 1 hour or until sizzling and golden brown.
