Slice the half fennel bulb with a sharp knife or mandoline into paper-thin slices. Remove the core from the half green apple and slice the flesh into thin slices. Finely chop the fresh dill and flake the smoked mackerel fillet with your hands into sturdy pieces.
Make the dressing by mixing the extra virgin olive oil and lemon juice in a small bowl. Season with Celtic sea salt and freshly ground pepper. Whisk together briefly until the oil and juice are well combined.
Place the sliced fennel and apple in a large bowl and pour the dressing over it. Toss well so that the flavors distribute immediately. Then add the pieces of mackerel and finely chopped dill to the bowl. Now toss everything very gently to prevent the fish from breaking apart too much.
Distribute the mackerel salad directly onto plates or transfer to a shallow serving platter. Serve immediately so that the apple and fennel retain their crispy texture.