First wash, and cook potatoes with their skin. Peel, and mash one immediately, and let the rest (for the filling) cool down.
Preheat oven to 390°F
In a bowl with mashed potato, add margarine, flour, baking powder, salt, and pepper, and knead with hands to get a smooth dough. Add half of chopped up leek, knead it in, then transfer the dough to a medium sized spring form pan, spread it around to cover pan bottom with it.
Peel and slice the remaining potatoes, or chop them up, and place them on top of the dough, along with half of chopped up leek. Sprinkle with 1/4 t sea salt and pepper.
Mix eggs with sour cream, add a pinch of salt, and feta cheese. Pour this over the potatoes, and place the pie in the oven. Bake for about 40 minutes, depending on the oven.
Cool a little, then remove the spring form, and serve. Best if eaten right away, as most potato dishes.