Bring the fish fumet to the boil in the first saucepan.
Drop in the frogs’ legs and as soon as the liquid comes back to the boil take them out. Put them on one side wrapped in a napkin.
Put the white wine, chopped shallot, tarragon, salt and pepper in the second saucepan. Bring to the boil and reduce, uncovered, until the white wine has all but evapor