Lightly season fish with marinade for 20-25 minutes. Heat butter and corn oil in a non-stick saucepan. Add garlic, bay leaves and onion and fry until fragrant and onion starts to brown. Add mushrooms and potato cubes and fry for a while. Pour in milk and add the thickener. Bring to a low simmering boil, stirring constantly to prevent burning. Turn off the heat when the mixture thickens. Adjust the seasoning, add green peas and fish slices.
Fill two small pie dishes or a big pie dish with the prepared filling. Roll out the puff pastry so it is large enough to cover the pie dish. Trim the edge or allow the pastry to overhang slightly. Brush the top with the beaten egg. Bake in preheated oven at 190°C for about 25-30 minutes or until pastry top is golden brown. Remove from oven and serve immediately.