Normandy fish stew

Name: Normandy fish stew
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: French
Ingredients: 500g mussels cleaned and debearded (discard any that don't close when tapped on the sink); 300ml cider or white wine; large knob of butter; 2 leeks cleaned and sliced; 100g baby button mushroom halved; 150ml crème fraîche; 4 fillets skin-on sustainable white fish (plaice would be perfect); small bunch flat-leaf parsley chopped