Cheesy Fish Chowder Pie

Name: Cheesy Fish Chowder Pie
Ingredients: 400 g Fish Pie Mix (pieces of smoked haddock, cod, salmon etc) (fresh or frozen); 4 Peppercorns; 1 Leek (sliced); 1 x 325 g Tin sweetcorn; 200 ml Fish stock (from poaching the fish); 200 ml Semi-skimmed milk; 25 g Plain flour; 25 g Butter; 1 tablespoon Clotted cream (optional, I just happened to have it); 1 teaspoon Whole grain mustard; 90 g Extra mature Cheddar cheese; 10 Cornichons (or to taste); 1 x 320 g Pre-rolled puff pastry sheet; 1 Egg (whisked to use as egg wash)