Lattice-topped fish pie | Sainsbury`s Magazine

Ingredients

• 100g soft butter
• 2 leeks, split lengthways, rinsed and thinly sliced
• 400ml fish, chicken or vegetable stock
• 200ml milk
• 40g plain flour, plus extra for dusting
• finely grated zest of 1 lemon
• freshly grated nutmeg, to taste
• 2 tbsp chopped flat-leaf parsley leaves
• 1 tbsp chopped tarragon
• 240g skinless and boneless salmon fillet, cut into 4cm chunks
• 250g boneless, skinless smoked haddock, cut into 4cm chunks
• 225g raw large prawns, defrosted and peeled
• 250g all-butter puff pastry, defrosted if frozen
• 1 egg yolk