Start by preparing the vegetables. Rinse the fennel and half cucumber thoroughly under running water. Slice both ingredients paper-thin using a sharp knife or mandoline and place them together in a large mixing bowl. Next, remove the skin and bones from the smoked mackerel. Flake the fish flesh with your hands into rough pieces and add to the bowl with the finely sliced vegetables.
Make the dressing by mixing the two tablespoons of olive oil with the freshly squeezed lemon juice. Season this mixture with a pinch of salt and freshly ground pepper. Pour the dressing over the fennel, cucumber and mackerel. Toss gently, ensuring the ingredients are well combined but the fish pieces remain intact.
Finely chop the fresh dill. Sprinkle most of the dill over the salad and toss it through lightly. Reserve a small amount of dill for garnish. Taste the mixture and add a little extra salt, pepper or lemon juice if needed to balance the flavours.
Divide the mackerel salad among plates or transfer to a serving dish. Sprinkle the reserved dill over the top just before serving. Serve immediately.