In a large saucepan, cover potatoes and salt with water then boil until soft, about 20 minutes. Drain then mash with 30g of butter and 3/4 cup milk. Season with salt and pepper, to taste. Set aside.
Melt remaining butter in a saucepan over medium heat then stir in the leak and asparagus. Cook until tender then stir in the flour until well mixed. Slowly whisk in the 1 1/2 cups milk then the cream. Bring to a boil and continue to whisk until the mixture thickens, about 3 minutes. Stir in the frozen vegetables, salmon and dill. Set aside to cool.
Preheat oven to 180 degrees C. Press pastry into a pie dish then bake in oven for 10 minutes. Remove from oven, pour in salmon filling, top with mashed potatoes then bake in preheated oven for 25 minutes. Allow to cool 10 minutes before serving.