2 cans sauerkraut
2 tbsp butter
1/2 lb bacon, cut into pieces
1 med onion, diced finely
pepper to taste
Dump sauerkraut and juices into a large saucepan. Add butter. Cook, covered, over medium-low heat for an hour or two (the longer you cook it, the more mellow the flavor. If you want to cook it longer, it's fine, you'll just need to add some water so it doesn't burn.)
Place sauerkraut in a colander and push out as much liquid as possible. Leave draining while you prepare the bacon. In a large frying pan, cook bacon until fat is rendered and the edges turn crispy. Add onion, cook until translucent. Add sauerkraut and pepper, stirring until heated through. Return everything to the colander, and push out as much liquid as possible. Cool, draining as thoroughly as possible.
When adding this filling to pierogi, squeeze out any additional water while forming balls of filling.