Wash and thoroughly dry the rocket. Check the smoked mackerel fillet for any remaining small bones, remove the skin and flake the fish into large pieces. Wash the radishes, remove the leaves and root, then slice them into very thin slices.
Whisk together the lemon juice, Dijon mustard, sea salt and black pepper in a small bowl. Pour in the olive oil in a thin stream and continue whisking until a smooth, emulsified dressing forms.
Combine the rocket and radish slices in a large mixing bowl. Pour about two-thirds of the dressing over the salad and toss everything so that all ingredients are coated. Add the flaked mackerel and gently fold it through so that the pieces of fish remain intact. Taste and add the remaining dressing if needed.
Distribute the salad to the plates immediately after tossing to prevent the rocket from wilting. If desired, grind a little extra black pepper over the top.