In a large pot, melt the butter. Add the sliced leek and sauté until soft, about 5 minutes. The leek should not brown.
Pour in the broth, add the diced potatoes, and bring to a boil.
Simmer the soup for 10-15 minutes until the potatoes are almost soft.
Cut the salmon into cubes (approximately 2 cm).
Add the salmon to the simmering soup and cook over low heat for 5-7 minutes until the salmon changes color to opaque and is cooked through. Do not stir too much to prevent the salmon from breaking apart.
Pour the cream into the soup. Let the soup come to a gentle boil (it should not boil vigorously).
Add most of the chopped dill, salt, and freshly ground white pepper to taste. Stir and immediately remove from heat.
Serve Lohikeitto hot, garnished with fresh dill in each portion, and traditionally accompanied by Finnish rye bread or hardtack.