Parboil cabbage in boiling water for ten to fifteen minutes. Rinse, drain, cool, & thoroughly wring moisture from cabbage & sauerkraut; set aside. The drier the cabbage & sauerkraut the less time required to complete cooking. Saute onion & garlic in 2 Tbls. Butter. Add cabbage & sauerkraut & remainder of butter; cook for approximately one half hour or until cabbage is soft & mixture is "reasonably" dry. It is important that the mixture not be too moist in order to prevent wetting dough during the filling process wet dough will cause pierogi to break open during cooking. Allow mixture to stand in refrigerator (in covered bowl) for twenty four hours prior to filling pierogi dough.