Preheat the oven to 110°C/gas mark ½. Place the sea trout fillets, 75 ml of cream, a little lime zest and 2 egg whites to a food processor and blitz to a fine purée, check seasoning, then chill the pureé for one hour.
Next make the croquettes, first mix the smoked eel and horseradish into the cold, mashed potato and season. Take small handfuls of the mixture and roll into small 2cm by 1cm sausage shapes, then chill. Whisk one egg with a little milk, in a small bowl, and place the flour and breadcrumbs into 2 more bowls. Dip the chilled potato mixture first in the flour, then in the egg, then in the breadcrumbs, to coat completely. Heat a little oil in a frying pan, and fry the croquettes until golden brown. Keep warm.
Poach the sea trout mousse by simmering a little seasoned water with a few flavourings (star anise, etc) and forming ‘quenelles’ with two tablespoons, spooning inside each other to form an oval rugby ball shape or put the mixture into a piping bag and piping 5cm lengths instead. Then cook these for 4 to 5 minutes in the water with a little buttered greaseproof paper to cover and ensure even cooking. Once cooked, remove from the liquid and drain on kitchen towel.
To Make the Watercress Purée
First wilt the watercress stalks and leaves in a little unsalted butter. Add a splash of cream. Blitz to a purée and season.
To plate, warm the croquettes and quenelles of sea trout through in the oven for 3 to 4 minutes. Place a spoonful of warm watercress purée in the centre of a warmed plate, with the croquettes at alternative ‘thirds’ on top of the purée and the quenelles in between. Garnish with a little mustard cress and a few peashoots. Serve immediately.