1. Wash the fish and pat dry with kitchen paper. Finely chop 1 tablespoon mint leaves and stir into the soured cream - add a little squeeze of lemon juice and a grinding of pepper to taste.
2. Cut out and lightly oil four sheets of aluminium foil large enough to parcel up each fillet.
3. Lay a fillet on each sheet, season lightly with a grating of nutmeg, a pinch of salt and a grinding of pepper, and sprinkle over a few bruised mint leaves and a squeeze or two of lemon juice.
4. Bring up the edges of the foil and crimp into loose parcels. Steam for 8 to 10 minutes, accordingto the thickness of the fillets.
5. Meanwhile, gently warm the soured cream and mint mixture in a small pan, it should be just lukewarm.
6. The fish can be served in their parcels on warm plates — neatly fold back the foil and pour the lukewarm soured cream mixture around the fillets so that it mingles deliciously with the cooking juices.
7. Garnish with whole mint leaves and serve at once.