Preheat the oven to 375°F (190°C).

In a large saucepan, melt the 50g of butter over medium heat. Add the onion, celery and carrots and cook for 5-7 minutes, stirring occasionally, until the vegetables have softened.

Sprinkle in the flour and stir to coat the vegetables. Cook for 2 minutes, then gradually whisk in the milk, stock, wine or sherry, Dijon mustard and thyme. Season with salt and pepper.

Bring the sauce to a simmer and cook for 5-10 minutes, stirring constantly, until thickened.

Remove the sauce from the heat and stir in the chunks of white fish and chopped smoked salmon. Transfer the mixture to a 9x13 inch baking dish.

Place the potato chunks in a large saucepan and cover with cold water. Bring to a boil and cook until tender, about 15-20 minutes. Drain the potatoes and mash them with the 50g of butter and 50ml of milk until smooth and creamy.

Spread the mashed potatoes over the fish mixture in the baking dish, using a fork to create peaks and ridges on the surface.

Sprinkle the grated cheddar cheese over the top of the potatoes.

Bake the fish pie in the preheated oven for 35-45 minutes, until the topping is golden brown and the filling is bubbling.

Let the pie rest for 5-10 minutes before serving.