Chop the meat in a meat grinder. Chop the onion or finely, or chop it too in a meat grinder, but choose the nozzle with the largest holes. Add salt, pepper, onion, a little water, butter softened at room temperature to the minced meat and knead. You need to add a little water and knead for a few minutes. The longer we knead the minced meat, the more tender our filling will turn out later. The amount of water in the cooking process is controlled each time, because the meat can be completely different in quality. As a result, our minced meat should be quite juicy, but not liquid.