Remove the skin and any bones from the smoked mackerel. Flake the fish with your fingers or two forks into large pieces and place them in a spacious mixing bowl. Rinse the half green apple, remove the core and chop the flesh into small, equal pieces.
Add the apple pieces to the bowl with the flaked mackerel. Then add the mixed greens to the mixture. Gently toss the ingredients together so the fish remains intact and the salad leaves aren't unnecessarily bruised.
Drizzle the salad with lemon juice. The acidity of the lemon balances the rich texture of the smoked mackerel. Toss the salad one more time gently so the juice is well distributed over the fish, apple and greens.
Divide the mackerel salad over a plate or deep dish. Serve immediately so the apple pieces taste fresh and the mixed greens retain their crispy texture.