1. In a large pot, melt the butter over moderately low heat. Add the onions and carrots; cook, stirring occasionally, until the onions are translucent, about 5 minutes. Add the curry powder and cook, stirring, for 1 minute. Add the broth and water. Bring to a boil. Reduce the heat; simmer until the carrots are tender, about 10 minutes.
2. Discard any mussels that are broken or do not clamp shut when tapped. Put the wine and the mussels in a large stainless-steel pot. Cover and bring to a boil. Cook, shaking the pot occasionally, just until the mussels open, about 3 minutes. Remove the open mussels. Continue to boil, uncovering the pot as necessary to remove the mussels as soon as their shells open. Discard any that do not open. When the mussels are cool enough to handle, remove all but twelve of them from their shells, holding them over the pot to catch all the juices.
3. Puree half of the carrot mixture in a blender or food processor. Return the puree to the soup pot and stir in the salt. Carefully pour the mussel-cooking liquid into the soup, leaving any grit in the bottom of the pot. Add the shelled mussels to the soup and cook over low heat just until warmed through, about 1 minute. Serve the soup topped with the reserved mussels in their shells.
Wine Recommendation: The perfect choice with curry spices is a refreshing acidic white wine. Try a bottle of the classic shellfish wine, Muscadet de Sèvre-et-Maine from the Loire Valley in France.