In a large pan, place the chopped bacon over medium heat. Cook, stirring occasionally, until crisp and the fat has rendered. Add the chopped onion and continue to cook until golden.
Bring a large pot of salted water to a boil. Add a few of the pierogi, taking care not to overcrowd, and stir gently to separate from the bottom. Cook until the Pierogi float to the top and the edges are tender, about 3-5 minutes. Use a slotted spoon to remove, drain, and repeat with remaining Pierogi.
In a large skillet, melt the 2 tablespoons (30 grams) butter over medium heat if pan-frying.
Add enough boiled Pierogi to cover in a single layer (do not overcrowd) and cook until lightly golden on both sides. Repeat with remaining Pierogi, adding more butter as needed.
Toss the boiled or pan-fried Pierogi with the bacon and onions and serve immediately with sour cream if desired.