Preheat the oven to 190C/375F/Gas 5.
In a bowl, mix together the curry powder and the salt until well combined. Dredge the monkfish pieces in the powder, then season with freshly ground black pepper.
Heat the oil in an ovenproof frying pan over a medium heat, add the monkfish pieces and fry for 3-4 minutes on each side, or until just golden-brown on both sides.
Add half of the butter to the pan, then transfer to the oven and cook for a further 4-5 minutes, or until the monkfish is cooked through. Remove from the oven and squeeze over the lemon juice.
Heat the remaining butter in a large frying pan over a medium heat until foaming. Add the shallots and fry for 2-3 minutes, or until softened.
Add the bay leaf, thyme, tarragon and saffron strands and stir well.
Add the white wine, stir well and bring the mixture to the boil. Reduce the heat to a simmer and simmer until the volume of liquid has reduced by half.
Add the cream and the fish stock and bring the mixture to the boil.
Add the mussels, cover the pan with a lid and cook for 4-5 minutes, shaking the pan occasionally, until the mussels have opened. Discard any mussels that remain closed.
Remove the mussels from the pan using a slotted spoon and set aside. Continue to simmer the cooking liquor until it has thickened slightly.
Add the mustard and the chopped tarragon and stir until well combined. Season, to taste, with salt and freshly ground black pepper and continue to simmer the mixture for a further 1-2 minutes.
To serve, slice each of the monkfish pieces into two medallions. Place two of the monkfish medallions into the centre of each of four serving bowls. Remove half of the mussels from their shells and divide them among the bowls. Divide the remaining mussels between the bowls. Spoon over the tarragon and mustard sauce and serve immediately.
