Parboil cabbage in boiling water for ten to fifteen minutes. Rinse, drain, cool, and thoroughly wring moisture from cabbage and sauerkraut; set aside. The drier the cabbage and sauerkraut the less time required to complete cooking. Saute onion and garlic in 2 Tbsp. Butter. Add cabbage and sauerkraut and remainder of butter; cook for approximately one half hour or until cabbage is soft and 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.