Here's how you do it

1. First, prepare the mussels. Take them out of the fridge 30 minutes before cooking, then add them to a bowl of fresh water. Discard any mussels that have damaged shells or those that don't close when pressed. Drain and set aside.
2. Pour a glug of olive oil into a large pan on a medium heat and add the onion, fennel, carrot and celery. Sweat for 10 minutes, until soft.
3. Meanwhile, put a frying pan on to a high heat and add olive oil. Slice the fish fillets in half and season with salt and pepper. Fry for 4 minutes, skin-side down. Turn and cook for 1 more minute, then remove from the heat.
4. Add the garlic and red wine vinegar to the vegetables, cook for a minute.
5. Turn up the heat and add the mussels, cover the pan with a lid. Cook for 2-3 minutes, until all the mussels have opened. Discard any mussels that haven't opened.
6. Add the cannellini beans to heat through for a minute. Stir in the cream and season with salt and pepper.
7. Serve the stew with the fish placed on top, sprinkled with chopped parsley.