Melt butter in a large pot on the stovetop over medium heat. Add in onions and sauté for about 4 minutes. Mix in flour and continue to cook for an additional minute, stirring constantly.
Stir in chicken broth, potatoes and bay leaf and bring to a boil. Cover and reduce heat to minimum. Simmer for 15 minutes. Take out the bay leaf.
Add salmon and continue to simmer, uncovered, for about 5 minutes. The salmon will cook in the liquid.
Stir in heavy cream, 2 tablespoons of chives, vinegar, nutmeg, cayenne pepper, salt and pepper. Turn heat up to medium for about 5 to 7 minutes, stirring often.
If you would like a thicker soup, add cornstarch to a bowl. Add enough of the soup to the bowl to make a smooth paste. Add to soup post and stir.
Garnish each bowl with chives and season with salt and pepper, if desired.