Put a large saucepan over a medium heat and add the butter, onion and garlic. Fry off for about ten minutes until everything is lovely and soft. Crumble in the veg stock cube and add the lemon juice.
Increase the heat, throw in the mussels and the wine and put the lid on the pan. The wine will make steam and start to cook the mussels.
Once the mussels are nearly all open (about two or three minutes) turn the temperature down to low and add the double cream, parsley and black pepper.
Slowly bring up the temperature until the sauce starts to bubble – be careful when adding the cream as if it’s too hot it can split.
Once the mussels are all open, they are ready – don’t leave them on the heat as they will overcook and go rubbery. Discard any that stubbornly refuse to open.
Serve with a wedge of lemon and lots of crusty bread (and not forgetting more butter).