How to Make It
In a large heavy pot, heat the butter over medium heat until hot, stir the onions, 3/4 teaspoon salt, and 1/2 teaspoon pepper and cook, stirring occasionally, until golden, about 8 minutes.
Peel and dice the potatoes. Stir in the potatoes and stock bring to a gentle boil. Cook the soup, stirring occasionally, until the potatoes are very soft and starting to fall apart, 45 minutes to 1 hour. Stir in the cream and crumble the salmon into the soup. Bring to a simmer, then remove from the heat. Season the soup with salt and pepper to taste and serve topped with the dill fronds.