Sauté chopped leek and green onion in a little butter.
Cut fish into strips 2 inches wide and add to the frying pan.
When fish is half cooked, remove and set aside.
Put 2 or 3 heaping spoonfuls of sour cream in the pan, and stir it in under a low flame.
Add fish, spoon mixture on top, turn up heat and finish cooking.
Add salt and pepper, as necessary.