In a medium bowl, stir flour and salt together. Cut in butter using a fork until pieces are very small. Stir in water and mix until well blended. If dough is too sticky, stir in a little more flour until you can knead it on a floured surface. Knead dough for 2 to 3 minutes. Cover, and let rest for 10 or 15 minutes.

On a lightly floured surface, roll dough out to 1/4 inch thickness. Cut into circles using a cookie cutter or large glass. Set aside while you prepare the filling.

To make the filling, prepare mashed potato flakes according to package directions; set aside. Heat olive oil in a skillet over medium heat. Add onions; cook and stir until soft and transparent. Remove from heat and mix in mashed potatoes.

Place one tablespoon of pierogi filling onto each circle of dough. Fold circles over and pinch the edges to seal tightly so no filling can escape while they boil. They can be frozen at this time if you like.