Hake fish cakes with mustard middles

Name: Hake fish cakes with mustard middles
Ingredients: 450g floury potatoes, cut into chunks (we used Rooster potatoes); 1 bay leaf; a few peppercorns; 450g skinless hake fillet, cut into 4; bunch spring onions, finely shredded; 1 whole nutmeg; 3 tbsp plain flour, seasoned; 1 egg; 100g fresh breadcrumbs; 2l vegetable oil, for deep-frying; salad leaves, to serve; lemon wedges, to serve; 100g full-fat crème fraîche; 50g strong cheddar, grated; 1 egg yolk (reserve the white); 1 tbsp wholegrain mustard