Baked Cod Fish in Dill Cream Sauce With Duchess Potatoes

Name: Baked Cod Fish in Dill Cream Sauce With Duchess Potatoes
Ingredients: 800 grams potatoes; 25 grams butter; 75 ml whipping cream; 150 ml milk; 2 egg yolks; ½ - 1 tsp salt; ½ tsp ground white pepper; 600 grams cod fillet; Salt and pepper for seasoning; 3 tbsp all-purpose flour; ½ tbsp chicken boulion; 200 ml cold water; 100 ml milk; 200 ml whipping cream; 1 tbsp fresh dill, chopped (or frozen dill); Finely grated lemon peel from ½ a lemon + a little lemon juice (½ tsp); Salt & ground black pepper, adjust to your taste; 100 grams peeled boiled prawns; Fresh dill (optional)