Cut the cooked beetroot into chunks and the apple into thin strips. Toss the apple immediately with the lemon juice so it doesn't discolour and retains its fresh flavour.
Stir the yoghurt with the horseradish and some black pepper to make a dressing. Horseradish gives a sharp kick that pairs well with the rich, smoked fish.
Distribute the mixed leaves over a plate and arrange the beetroot and apple on top. Break the trout into large flakes over the salad and spoon the horseradish yoghurt over. Scatter dill fronds on top. Beetroot provides nitrates and fibre, the trout omega-3 fatty acids and protein, together a lunch that keeps your energy stable.