In a large bowl, mix flour, oil, salt, and eggs until a dough forms.

Roll the dough out on a lightly floured surface to about 1/8 inch thickness.

Cut the dough into 2x2 inch squares.

In a separate bowl, combine mashed potatoes and cheese for the filling.

Place a spoonful of potato and cheese filling in the center of each square.

Fold the square in half diagonally to form a triangle.

Press firmly around the edges to seal.

Bring a large pot of water to a boil.

Gently drop the pierogies into the boiling water.

Cook for about 5-7 minutes, or until they float to the surface.

Remove the pierogies from the water with a slotted spoon.

In a large skillet, melt butter and add chopped onions.

Fry the pierogies in the butter and onion mixture until golden brown and crispy.

Serve hot.