Combine all sauce ingredients (red curry paste through coconut milk) in a large stock pot and stir well, making sure sugar and curry paste are nicely dissolved.
Bring to a boil over high heat and boil for 2 minutes.
Add mussels. Cover and cook, stirring every now and then, until mussels open - 6 - 8 minutes.
Remove from heat and discard any unopened mussels. Add in the cilantro and toss to combine.
Dump into a nice big serving dish and serve with crusty bread and lime wedges. I think it's fun and communal to set out a big basket of crusty bread and just let your guests all hover over the serving dish, dunking bread and picking out mussels!
NOTE: The sauce is phenomenal - you won't want to miss a drop; you could also pour over steamed brown or white rice for a bright, hearty and satisfying dish.