Arctic char with creamy horseradish sauce

Ingredients

• 1/2 leek
• 1 onion
• 3 bay leaves
• about 6 peppercorns
• a few sprigs of fresh dill (leftover dill, or use dried dill)
• 200 ml dry white wine
• 1/2 liter fish stock (store-bought or make your own)
• pepper and salt to taste
• 2 tablespoons grated horseradish
• 100 ml cream
• cornstarch as a thickening agent
• 4 arctic char fillets or any other fish fillet for serving