Preheat the oven to 190°C.
Mix curry powder and salt in a bowl, dredge monkfish pieces in the mixture, and season with pepper.
Fry monkfish in oil in an ovenproof pan over medium heat for 3-4 minutes per side until golden-brown.
Add half the butter to the pan and roast in the oven for 4-5 minutes until cooked, then finish with lemon juice.
Sauté shallots in the remaining butter in a separate large pan until softened.
Stir in bay leaf, thyme, tarragon sprig, and saffron.
Pour in white wine, bring to a boil, and reduce by half.
Add cream and hot fish stock, then return the mixture to a boil.
Add mussels, cover, and cook for 4-5 minutes until shells open.
Remove mussels with a slotted spoon and simmer the liquid until slightly thickened.
Stir in mustard and chopped tarragon, season, and simmer for 2 minutes.
Serve monkfish medallions topped with mussels and the tarragon sauce.