Sift flour and salt, then rub in butter chunks for the pastry.
Mix in egg yolks and water to form a firm dough, then chill for 20 minutes.
Roll dough to 20x60cm and fold into thirds like a letter.
Rotate 90 degrees, repeat rolling and folding twice more, then chill.
Simmer stock, lentils, onion, fennel, and garlic for 30 minutes until liquid evaporates.
Preheat oven to 200°C and line a tray with parchment.
Combine spices, aromatics, and fish sauce, then coat fish and marinate.
Whisk pancake ingredients to form a smooth batter.
Fry two pancakes in olive oil for one minute per side.
Roll 400g of pastry to a 50x15cm rectangle on the tray.
Layer one pancake, fish fillet, lentils, and the second fish fillet on the pastry.
Roll second pastry piece to 60x20cm.
Seal fish in pancake, brush pastry edges with egg white, and cover with top pastry.
Trim into a fish shape, score with scales, and brush with egg white.
Bake for 25 minutes until golden-brown.