Place 180g smoked salmon, double cream, horseradish sauce and lemon juice into a food processor and blend to a smooth purée.
Heat the chicken stock in a saucepan and bring to a simmer. Squeeze out any excess liquid from the gelatine, then add to the chicken stock and stir until dissolved.
Whisk the chicken stock into the smoked salmon purée until well combined. Season to taste with freshly ground black pepper.
Either pour into one large bowl or 6 individual ramekins, top with the remaining chopped smoked salmon pushing then into the mousse slightly and sprinkle with the dill. Place in the fridge and chill for two hours, or until set.
For the melba toast, grill both sides of the bread until golden brown, then cut the toast slices in half horizontally. Grill the soft inner sides until golden brown, and then cut the toast slices into quarters.
Serve the smoked salmon mousse with freshly made melba toast.