In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.

Add the diced potatoes and fish stock (or water) to the pot. Bring to a boil, then reduce heat and simmer until the potatoes are tender, about 15 minutes.

Once the potatoes are tender, add the cubed salmon to the pot. Cook for an additional 5 minutes until the salmon is cooked through.

Stir in the heavy cream, dill, salt, and black pepper. Heat through but do not boil.

Ladle the soup into bowls and garnish with fresh dill and lemon slices. Serve hot.