Preheat your oven to 400°F (200°C).
In a large skillet, melt the butter over medium heat. Add the diced onion and cook until softened, about 5 minutes. Stir in the minced garlic and cook for an additional minute.
Add the frozen peas and diced potatoes to the skillet, then pour in the fish stock. Bring to a simmer and cook for about 10 minutes, or until the potatoes are tender.
Stir in the heavy cream, dried dill, lemon juice, salt, and pepper. Allow the mixture to simmer for another 5 minutes.
Gently fold in the white fish and smoked salmon, cooking for an additional 2-3 minutes until the fish is just cooked through. Remove from heat.
Pour the fish mixture into the pie crust and sprinkle the shredded cheddar cheese on top.
Cover with a second pie crust, crimping the edges to seal. Cut a few slits in the top crust to allow steam to escape. Brush the top with the beaten egg.
Bake in the preheated oven for 25-30 minutes, or until the crust is golden brown.
Let the pie cool for 5-10 minutes before slicing and serving.