Classic Fish Pie

Classic Fish Pie Ingredients: For the filling: 1 ½ lbs (700g) mixed fish fillets (e.g., cod, salmon, smoked haddock), skin removed and cut into chunks 1 ½ cups whole milk (plus extra for mashing potatoes) 1 onion, peeled and quartered 1 bay leaf 2 tbsp butter 2 tbsp all-purpose flour 1 cup frozen peas 1 tbsp chopped fresh parsley Salt and pepper to taste For the mashed potato topping: 2 lbs (900g) potatoes, peeled and cut into chunks 3 tbsp butter ½ cup milk (or cream) Salt and pepper to taste Optional: ½ cup grated cheddar cheese