Marinate meat in wine with salt and pepper for at least one hour or overnight if possible.
Drain meat from wine and save the liquid for making the sauce.
Place oil in a frying pan and preheat to medium heat. Add schnitzel to frying pan to seal and flip once.
Move the schnitzel to another non-stick pan.
Chop onion and add to frying with remaining oil. Add more oil if needed.
Saute the onion until brown and then move to the other pan holding the schnitzel.
Pour the leftover wine over top of the meat. Add more wine if needed so the schnitzel is fully covered.
Cook meat on low heat for about 1 hour, check occasionally that it doesn't stick to the bottom.
In the meantime, using a separate bowl, combine 1 cup cold water with cornstarch and sour cream. Mix altogether to dissolve.
Once meat is almost cooked fully, cover the cornstarch mixture over top of meat.
If not creamy enough, add more dissolved cornstarch in cold water. If too thick, add water.
Cook for another 10 minutes on low heat until the sauce boils.