Pour 1 cup (250 ml/8 fl oz) of water into the lower compartment of a microwave steamer.
Put the potatoes into the upper compartment, cover and microwave on HIGH for 9 minutes.
Cut the fish into pieces and put in a dish with 3 (2) tablespoons of water.
Cover.
When the potatoes are cooked, take out of the steamer and leave to stand for 5 minutes.
Put the trout into the oven and microwave on HIGH for 6 minutes.
Leave to cool.
Put the butter into a bowl and soften in the oven for 30 seconds on HIGH.
Quarter the avocado, remove the stone, and peel 3 of the quarters.
Wrap the fourth quarter in saran wrap (cling film) and set aside.
Drain the fish and remove the bones and skin.
Puree the fish in a food processor.
Add the potatoes, the three peeled avocado quarters, butter and three-quarters of the lemon juice.
Liquidize once more.
Add a pinch of cayenne pepper and generous pinches of salt and pepper.
The mousse should be highly seasoned.
Divide the mixture between six glass serving bowls.
Smooth the surface and chill in the refrigerator for at least 4 hours.