Place the cleaned mussels in a large saucepan.
Add the onion and boiling water.
Cover the pan and cook over a high heat for 2-3 minutes or until the mussels have opened. Shake the pan while cooking.
Remove the pan from the heat, lift out the mussels, discarding any that have not opened.
Set the mussels aside. Strain the cooked liquid through a fine sieve.
Add the potatoes.
Add the mussels and milk to the soup.