Melt butter in a large soup pot or Dutch oven over medium-high heat. Add the sliced carrots and leeks, then season with Kosher salt and stir. Cook until softened.
Add the cubed potatoes and seafood stock, then bring to a boil, reduce the heat to medium, and cook until the potatoes are tender.
When the potatoes are fully cooked, add the fresh salmon and creme fraiche. Stir well, cover the soup, then simmer until the salmon is fully cooked. Stir in 1 cup chopped dill, taste and adjust seasoning, then divide between bowls. Serve.