In a small saucepan, bring the water, butter and saffron to a boil. Transfer to a blender, add the 30 nasturtiums and lime juice and blend until smooth. Strain through a fine sieve and season with salt.
In a large saucepan, combine the mussels and wine. Cover and cook over high heat until the mussels open, about 5 minutes. Stir in the nasturtium broth. Spoon the mussels and broth into bowls, garnish with nasturtiums and serve.