Saute the onions with some of the butter in a pan. Peel the potatoes and cut them into thin slices. Boil them in salted water for 5' - 8' and drain well. Mix 2/3 of the cheese with parsley, salt and pepper.
Prepare the Bechamel sauce according to the package directions or to our basic recipe. Pour a layer of Bechamel sauce in a greased form and then a layer of potatoes. Add above some onions and a little bit of the cheese-parsley mixture. Repeat with one - two more layers in the same way.
Add to the remaining sauce the eggs, the yogurt, the remaining cheese, salt and pepper and spread it on the surface of the food. Scatter over the remaining butter and bake in the oven at 200° C, 40'-45', until it gets gold-browned.