Prepare the mussels: Discard any mussels with broken shells. Rinse the rest in a sieve under cold running water. Pull out any “beards” or seaweed. If any mussels remain open, tap them gently with a spoon- if they don’t close, throw them away.
Heat the butter or olive oil in a deep pan over medium heat.
Add the celery and chicory and cook for 2–3 minutes until slightly softened.
Stir in the garlic and cook for another minute until fragrant.
Pour in the sherry, vermouth or wine along with the same amount of water.
Bring to a boil and reduce slightly for 2–3 minutes.
Add the mussels to the pan, cover with a lid, then steam for about 5 minutes, shaking the pan once or twice.
The mussels are ready when their shells have opened. Discard any that remain closed.
Serve with chopped parsley and season lightly with salt and pepper if needed.
Serve immediately with crusty garlic bread, French fries, or a crispy salad.