Remove the skin from the smoked mackerel and divide the fish flesh into large chunks; watch out for any bones. Cut the cucumber into thin half-moons and the avocado into cubes.
Distribute the mixed greens over a plate and arrange the cucumber, avocado and mackerel pieces on top. Sprinkle some finely chopped dill over everything.
For the dressing, whisk together the olive oil, mustard, lemon juice, salt and pepper until smooth. Spoon the dressing over the salad just before serving, so the avocado stays creamy and doesn't discolor.