Bring the chicken stock to the boil in a saucepan.
Add the potatoes and cook for 5 minutes, or until nearly tender.
Add the pearl onions and cook for a further two minutes.
Add the asparagus and cook for another two minutes, or until all the vegetables are tender.
Remove the vegetables with a slotted spoon and set aside.
Simmer the stock until the volume of liquid has reduced by one-third.
Check the mussels: if any are open, give them a tap on the work surface and if they don't close, throw them away.
Add the clams to the broth, cover and cook for 1-2 minutes, or until all the clams are open.
Whisk in the butter, then return the vegetables to the pan and warm through.
Season with salt (if needed) and freshly ground black pepper.
Serve the broth in bowls with crusty bread.