Make the dough: In a small bowl, mix warm water, salt and oil. In a separate large bowl, place flour, make a well in the middle, then pour the wet mixture into the well. Mix until smooth dough is formed.
Set aside to rest while making the filling.
Make the filling: fill a medium-large pot with water, add 1 tablespoon of salt, add potatoes and bring to a boil. Boil until tender.
Place boiled potatoes in a large bowl and mash.
In a medium sized pan, heat 2 tablespoons of oil, add chopped onions and cook until golden.
Add 3/4 of the onions to the mashed potatoes, season with salt and pepper and mix well.
Assembly: divide the dough into 4 pieces. Roll each piece into a 1/8cm thick. using a glass or a cookie cutter, cut 6-7cm circles. Fill each circle with 1 teaspoon of filling. Seal the dough and repeat with the rest of the dough.
Cook the vareniki: boil a large pot of salted water. Add the vareniki the boiling water and boil until floats. Drain.
Serve with the rest of the fried onions and with sour cream.
Notes:HOW TO STORE VARENIKI?Vareniki are at its best when they are hot straight from the boiling water. It is not recommended not eat them cold, but you also can fry leftover vareniki and get crispy vareniki.CAN WE FREEZE VARENIKI?Yes, you easily can freeze vareniki before cooking them in boiling water. Keep the varniki in an airtight container in the freezer up to 2 months. When ready to serve, cook them in boiling water until floats plus 2 minutes.