Seafood gratin with caramelised apples

Name: Seafood gratin with caramelised apples
Ingredients: 450ml/16fl oz fish stock (see tip); 400g/14oz white fish fillets (such as cod), skinned and cut into 2cm/¾in pieces; 200g/7oz scallops, removed from the shell and cleaned; 400g/14oz raw prawns, peeled and deveined; 50ml/2fl oz white wine; 150ml/5fl oz full-fat milk; 100ml/3½fl oz double cream; 50g/1¾oz butter, plus extra for greasing; 2 leeks, trimmed, halved lengthways and sliced; 50g/1¾oz plain flour; small handful fresh flatleaf parsley, roughly chopped; 80g/2¾oz emmental cheese, grated; 50g/1¾oz panko breadcrumbs; salt and freshly ground black pepper; crusty bread, cut into chunks, to serve; 3 tbsp unsalted butter; 2 tbsp golden caster sugar; 3 or 4 eating apples, peeled, cored and cut into 2cm/¾in chunks