Wash the fish fillets (you should have 4 from each fish) and pat dry with kitchen paper. Flatten them slightly (this will prevent them from shrinking when cooking) with the side of a knife blade and very lightly score them on the side that formerly had the skin on.

Cut the lemon in half, use one half for squeezing and slice the other for a garnish. Sprinkle each fillet with a little lemon juice and season with pepper.

Cover half of each fillet with prawns, then sprinkle a little of the chopped tarragon over the other half.

Fold the fillets in half and arrange them in a heavy poaching or frying pan. They should fit neatly in the pan so that the stock will easily cover them.

Now trim and wipe the mushrooms and halve or quarter, depending on their size. Dry fry them briefly in an oil brushed pan and then put them between and around the fillets.

Sprinkle the remaining tarragon all over, together with a squeeze or two of lemon juice and a grinding of pepper.

Pour in just enough warm stock to cover the fillets. Add more stock, or some white wine or water if necessary.

Bring slowly and gently up to just under simmering point and poach for 3 to 6 minutes, depending on the size and thickness of the fillets.

Using a spatula, carefully transfer the cooked fish to a warmed serving dish. Use a perforated spoon to fish out any stray prawns.

Arrange the mushrooms around the fillets. Keep warm.

Bring the poaching liquid up the boil and reduce by half, or until you have a good quantity of slightly thickened sauce.

Now take it off the heat and whisk in the fromage blanc, if using.

Pour this sauce all over the fish and serve immediately, garnished with slices of lemon and sprigs of tarragon.