Preheat the oven to 200 °C / 392 °F with top and bottom heat.
Bring 500 ml of water to a boil in a saucepan over high heat and add salt to taste. Add the fish fillet and simmer over medium heat for 3–5 minutes, then drain. Transfer to a baking dish.
Roughly chop the dill. Whisk together the sour cream, crème fraîche, and egg. Add the lemon quarter to taste. Season well with salt and pepper. If desired, add a splash of white wine.
Pour the sauce over the fish, ensuring it is completely covered. Bake at 200 °C / 392 °F for 20–25 minutes until the top is lightly golden.