Heat a large saucepan over a high heat for 1 minute, then reduce the heat a little and add the mussels and white wine. Cover and let the steam cook the mussels for 3-4 minutes. Drain and discard any mussels that haven't opened during cooking. Remove the mussels from their shells carefully and reserve to one side in a bowl.
Meanwhile, melt the butter in a saucepan over a medium heat and whisk in the flour to make a roux.
Cook the roux for 1-2 minutes, stirring occasionally, then whisk in the milk in a slow, steady stream until you have a thickened sauce. Simmer for a few minutes, then season to taste with the nutmeg and seasoning.
Add the salmon to the sauce and poach for 8-10 minutes over a low heat. Add the mussels back to the sauce to warm through. Spoon into serving bowls and garnish with black pepper and a sprig of parsley before serving.