Combine egg yolks and muffin mix and knead well. The mixture should be a smooth dough consistency.
In a medium saucepan, cook the onions in vegetable oil until soft, about 8 minutes. Remove from heat. Strain the onions, reserving the oil and the onions separately.
In a mixing bowl, mix mashed potatoes, 4 tablespoons of the onion flavored oil, salt, and pepper, to taste.
With rolling pin, roll dough to 1/8-inch thick, and cut into 2-inch square shaped pieces. Add dollop of potato filling to center, close the 4 corners of the dough, and pinch shut.
Bring a large pot of salted water to a boil. Add the Kartoffel and Verenika and boil gently for 10 minutes. Drain.
To serve, sprinkle with sauteed onions and croutons. Top with sour cream.