Cook mussels with thyme and 75ml of wine in a hot lidded pan for 2-4 minutes.
Strain the mussels and reserve the cooking liquid in a bowl.
Remove the mussels from their shells and discard the shells.
Season the halibut fillets with salt and pepper.
Fry the halibut in 25g of butter and olive oil for 2-3 minutes per side.
Saute shallots, fennel, and leeks in 25g of butter until softened.
Add curry powder, mushrooms, and pastis and cook for 3-4 minutes.
Add 175ml of wine, bring to a boil, and reduce until liquid is almost gone.
Deglaze the fish pan with reserved mussel liquid and reduce by half.
Combine the reduced mussel sauce with the vegetable mixture.
Stir in tomatoes, cream, saffron, and mussels; cook for one minute and season.
Mix in the chopped parsley and chervil.
Serve halibut fillets on top of the vegetable and mussel mixture.