Blend avocado, double cream, lime juice, and lemon juice into a smooth purée.
Simmer chicken stock, then stir in drained gelatine until dissolved.
Whisk stock into the avocado purée and season with salt and pepper.
Blend smoked salmon, double cream, horseradish, and lemon juice into a smooth purée.
Simmer more chicken stock and dissolve the remaining drained gelatine into it.
Whisk stock into the salmon purée and season to taste.
Portion salmon mousse into four 5cm rings, top with avocado mousse, and chill until set.
Whisk 4 tablespoons of double cream until thick and spread over the set mousse.
Whisk mustard, lemon juice, and olive oil, then toss with chopped avocado and seasoning.
Unmould mousses onto plates, garnish with salad leaves and dressed avocado.