Sauerkraut Pierogi Filling

Grind sauerkraut in a food processor. Transfer to a skillet; cook and stir over medium heat until softened, about 10 minutes. Drain in a colander.

Heat butter in skillet over medium heat; stir in onion and mushrooms. Cook and stir until onion has softened and turned translucent, about 10 minutes.

Combine sauerkraut with onion mixture, add salt and pepper, and cook for 2 minutes. Spread onto plate to cool completely.