Add the chopped leeks, sprinkle with salt and pepper, and saute on a low to medium heat for 10-15 minutes until they are soft.
Add the sour cream to the leeks, stir and cook for a minute. Remove from heat , taste and adjust seasoning if needed, then pour into a shallow baking dish (big enough to hold the fish fillet)
Meanwhile preheat the oven to 400 °F (200 °C).
Squeeze the juice of one lemon over the fish, and add salt and pepper liberally.
Slide the fish on top of the leeks, and bake for 15 minutes.