Peel and coarsely grate both the regular and sweet potatoes. Place the cut potatoes on to a clean tea towel or paper towels and lightly rub to dry them. Combine the potatoes and green onions with salt and pepper, tossing until they are well mixed.

In a large nonstick frying pan, or one coated with cooking spray, melt 1 tablespoon butter with 1 tablespoon oil over medium heat until hot. Add the potato mixture and spread it out evenly. Use a spatula to pat down the potatoes and pull in the edges to form a round thick pancake. Cover, reduce the heat to low and cook for 15 minutes.

Loosen the sides of the pancake and look underneath. If it is barely browned, continue cooking another 5 minutes or until it is browned on the bottom. Loosen the bottom. Lay a flat (no sides) cookie sheet or large plate overtop of the frying pan. Holding the plate over the frying pan, turn they frying pan over so that potato cake falls onto the baking sheet or plate.

Place the pan back on heat. Melt remaining butter with the oil. Keeping the browned side up, slide the potato cake back into the pan. Pat down and reshape the sides. Cook another 15 to 20 minutes on that side until it is browned on the bottom.