Bring 2 cups of water to the boil. Put the stock, shallot, garlic, lime leaves, and ginger in a saucepan, cover and let it bubble away at a moderate to high heat for 3 to 5 minutes or until you have a thickish, softish mess at the bottom of the pan. Check to make sure it isn't either burning or just getting too dry for comfort, and add a bit of boiling water if it is. Meanwhile, put the mussels in a sink filled with ice-cold water. Chuck away any that don't sink and then, as you remove them, make doubly sure by chucking away any that don't shut when you tap then sharply. I wouldn't expect you to have to throw many away, so don't worry unduly.
Add the chile pepper to the mixture in the pan. After about 30 seconds, or just long enough to get them out of the sink, add the mussels and then throw over them 1/2 cup boiling water, and then the lime juice, mirin, and fish sauce mixed together. Cover, give the pan a shake, and leave on a high heat for about 3 minutes, by which time the mussels should have steamed open. Add half the coriander, shake again, and pour into a noodle bowl. Sprinkle over the remaining coriander and eat.