For beurre d’escargot, combine all ingredients and season to taste. Set aside.
Drain potatoes and pat dry. Heat oil in a deep-fryer or saucepan to 140C. Add half the potato, cook until tender (3-5 minutes), drain on absorbent paper, repeat with remaining potato. Set aside. Increase heat to 180C.
Melt beurre d’escargot in a large heavy-based saucepan. Add shallot and herbs and cook over medium heat until shallot is soft (5-7 minutes). Add mussels, mushrooms and cider, cover, increase heat to high, shaking pot to distribute mussels evenly. Cook until mussels open (3-5 minutes), discarding any that haven’t opened, use a slotted spoon to transfer to a bowl and keep warm. Bring pan juices to the boil and cook until reduced by a third (5-7 minutes). Remove from heat, whisk in crème fraîche and season to taste.
Deep-fry potato until golden (3-5 minutes). Drain on absorbent paper and season to taste with sea salt. Spoon mussels into dishes, ladle over cooking liquid and scatter with parsley, Serve with chips and baguette, if desired.
