Mix the nuts and sultanas or currants together and spread the whole amount over 2 of the fish fillets. Put the lemon slices on top and then put the mint leaves on top of the lemon. Season with salt and pepper. Put a knob of butter in the centre of each fillet, then place the 2 remaining fillets on top.
Paint a sheet of filo pastry with melted butter and place one of the stuffed fish fillets on it. Wrap it neatly in a parcel, trimming off any excess pastry. Repeat this process until you have used up 4 sheets of pastry per fish. Paint again lightly with butter, place the parcels on a baking sheet and pop into a very hot oven for 12-15 minutes. When the pastry is golden brown, push a skewer into the centre of the fish and leave it for a moment or two; if it comes out hot, the fish is cooked.
For the sauce, melt the butter in a pan and stir in the flour to make a roux. Cook, stirring, for a minute or two, then gradually whisk in the lemon juice until you have a smooth sauce. Over a low heat, whisk the egg yolks rapidly into the sauce until it thickens, then stir in the mint. Taste and add a little salt and pepper if necessary.
To serve, divide the sauce between 4 plates, cut the fish parcels in half and place on top of the sauce.
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