Add rice. Cook together gently, covered, for 19 minutes.
Add boiling water and rest of ingredients. Continue cooking, covered, until liquid is absorbed.
When cool, stuff each mussel with the rice mixture and tie shut with string.
Place the mussels in layers in a saucepan, cover with boiling water and place a plate or small lid immediately on top of the mussels to hold them securely in place while they are cooking.
Put a lid on the saucepan. Cook over a very low fire for half an hour.
Drain, but leave mussels undisturbed to cool. Remove strings.