1. Wash and peel the potatoes and dice them roughly. Put them into a pot with a steaming basket, add enough water to cover the steaming basket and bring to a boil.
Wash and peel the sweet potato, cut into half and add it into the potatoes pot. Cook until the potatoes are tender, about 10 - 15 minutes. Don't worry if the the sweet potato is not tender yet, it will continue to cook in the oven.
2. Meanwhile soak the parsley, spinach, and leeks in cold water for a few minutes to remove sand or insects. Drain and dry them with a salad spinner or a clean kitchen towel. Chop finely only the parsley and set aside.
3. Preheat the oven to 380°F. Line a small Dutch-oven or a round oven-proof dish lined with baking paper (it makes it much easier to extract the cake from the pot).
Saute the spinach in a medium skillet over medium heat for 1 -2 minutes.
4. Put the potatoes in a bowl and mash them until they are smooth but lumpy. Break the eggs into the bowl and mix them with the mashed potatoes. Stir in the baking soda, white pepper, turmeric, salt, parsley and lemon juice and mix until combined.
5. Slice the sweet potato into ¼-inch rounds and place them onto the bottom of the lined dutch-oven. Drizzle the oil over them and pour half of the batter over it. Cover with a layer of spinach, then pour in the rest of the batter. Put the pot into the oven for 45 minutes or until the cake/frittata is golden brown.