Begin by preparing the ingredients. Flake the smoked mackerel and remove any remaining bones. Dice the half tart apple into small pieces. Whisk the mustard and olive oil together in a small bowl until a smooth dressing forms.
Take a large plate and spread the mixed leafy greens over it. In a separate bowl, mix the flaked smoked mackerel well with the apple pieces. Spoon this mackerel mixture onto the center of the greens.
Break the walnuts into rough pieces and scatter them over the salad. Drizzle the entire salad evenly with the prepared dressing so all components are lightly coated with the oil and mustard.
Serve the salad immediately after assembling. The quick preparation keeps the greens crisp and the apple provides a fresh bite to counterbalance the rich fish.