Scrub the mussels with a stiff brush and use a small, sharp knife to scrape off the beards and any barnacles. Wash well in cold water and discard any open mussels.
Heat the oil in a large deep saucepan, add the onions, garlic, ginger and chillies and cook over a gentle heat, stirring occasionally, for 5 minutes or until softened. Stir in the saffron with its infusing water, if using, or add 2 tablespoons of plain water, the ground coriander and cumin and cook for 1 further minute.
Add the salt and water to the pan, then stir in the mussels. Cover the pan and cook for about 5—6 minutes or until all the mussels have opened and cooked through. Discard any mussels that have not opened. Stir the lime juice and chopped coriander into the pan; taste and adjust the seasoning if necessary.
Ladle the hot mussels and the masala sauce into 4 warmed deep bowls and garnish each portion with a sprig of fresh coriander, if liked. Serve immediately.