Preheat butter and oil in skillet over medium heat.
Cut onion into thin slices. Add the sliced onion and some salt into the skillet.
Cook onions for 30 minutes, stirring occasionally until brown.
When the onions are almost finished, add the frozen pierogies to a separate skillet in a single layer. Cook for 7-8 minutes on each side or until golden brown.
After the onions are finished, allow to cool slightly. Then add to a food processor or blender with the sour cream. Puree until smooth and add salt to taste.
Add dip to a bowl and pierogies to a platter and top with green onions if desired. Serve warm and enjoy!