To make the dough: combine the wet ingredients into a bowl of a stand mixer, followed by the dry ingredients. Knead the dough for 3 minutes. Let rise until doubled (30-45 minutes.)
To make the filling: Saute the onion in 3 Tbsp of oil over medium heat for 2-3 minutes. Set aside. Boil the liver in a quart of water for 20 minutes. Drain and run through a meat grinder with the sauteed onion mixture (or chop in a food processor.) Stir into the mashed potatoes.
To Shape Piroshki: transfer dough onto a lightly floured surface. Form dough into a 34" long log and cut into 1-inch slices.
Slightly flatten dough piece and fill with 1 Tbsp of filling. Fold and pinch the edges shut. Place onto a floured tray.
Fry the Piroshki for 2-3 minutes on each side or until nicely browned. Drain on paper towels. Serve Warm!
