Clean mussels thoroughly, remove beards, and discard any broken or open shells that do not close.
Heat some butter in a large lidded pan and fry half the shallot and thyme until soft.
Add mussels and 100ml white wine, cover, and shake regularly until cooked.
Strain mussels and reserve the cooking liquid for the sauce, discarding any unopened mussels.
Remove mussels from their shells and set aside.
Blanch tomatoes in boiling water, peel, seed, and dice them.
Fry the remaining shallot in butter, then add sliced fennel and a pinch of saffron.
Pour in the reserved mussel liquid, reduce until thickened, then stir in cream and remaining butter.
Mix in samphire, shelled mussels, and tomatoes, then simmer for 3 minutes until tender.
Fry the seasoned fish skin-side down in oil for 5 minutes, then flip and cook for 1 minute.
Plate the sauce, top with the fish, and garnish with micro fennel.