Wash the mussels and pull off any ‘beards’ that are attached at the end of the shell. Discard any mussels that are open and don’t close when lightly pinched together, and any with damaged shells.
Place a large saucepan over a medium heat. Add the olive oil and butter. When the butter has melted and starts to bubble, add the leek, garlic and apple. Sauté until the leeks are tender and the apple has started to break down to a pulp.
Increase the heat and add the cider, cream and mustard, and season with salt and pepper. Bring to the boil, then throw in the mussels. Stir around, then cover the pan with a tight-fitting lid. Cook for 3 minutes, shaking the pan a couple of times, then check to see if the mussels have opened. If not, replace the lid and continue to cook for a further 1 minute, or until the mussels are all open.
Scatter over the parsley and divide between bowls. Discard any mussels that have refused to open.
Serve hot, with crusty bread to mop up the creamy cider broth. Flexible Mussels aren’t available all year round. If you can’t get hold of any but see some clams, then grab a net of those instead. They are smaller but just as tasty, and work well with this sauce.