Melt the butter in a 6-quart stock pot over medium-high heat. Add the onions and celery. Season with salt, cayenne, and black pepper, stirring, until the vegetables are soft and lightly golden, about 8 minutes. Add the bay leaf and garlic, stirring for 2 minutes. Add the stock and potatoes and bring the mixture to a boil. Reduce the heat to medium and simmer, uncovered, until the potatoes are very soft and the mixture is thick and creamy, about 1 hour. Remove the soup from the heat. Discard the bay leaf. With a handheld blender, puree until smooth. Slowly add the cream. Stir to blend. Reseason the soup. In a small mixing bowl, combine the salmon, red onions and chives. Drizzle the relish with enough oil to moisten. Season the relish with black pepper. To serve, ladle the soup into individual bowls. Garnish the soup with the relish.