Preheat the oven to 180°C (350°F).

Place salmon fillet on a parchment-lined baking sheet. Drizzle with 15 mL (1 tbsp.) olive oil, and season with salt and pepper.

Bake for 15 minutes. Remove from oven, remove skin from fillet, break it up into pieces, and set aside. 

Preheat oven to 200°F (400°C).

Cook potatoes in a pot of salted water until tender, about 8 minutes. Drain.

Add the milk, butter, salt, and pepper, and then mash or set aside a few chunks if you like. If needed, add more milk to obtain desired consistency. Set aside.

In a saucepan, sauté the onion and leek in 15 mL (1 tbsp.) olive oil until tender, about 8 minutes. Set aside.

In a large bowl, combine the mashed potatoes, onion-leek mixture, and reserved salmon pieces. Mix gently. 

Fill a 20-cm (8-in.) pie pan with the first pie dough round. Prick dough all over with a fork. 

Add potato and salmon filling. Cover with second pie dough round. Use a fork to seal edges and make a few incisions in top of pie. Brush with milk.

Bake for 40 minutes or until crust is golden brown.