Start by preparing the ingredients. Cut the quarter fennel with a sharp knife or a mandoline into paper-thin slices and remove the hard core. Take the smoked mackerel and flake the fish with your hands into large, bite-sized pieces. Carefully check that no small bones remain in the fish.
Take a large mixing bowl to assemble the salad. Put the paper-thin fennel and flaked mackerel in it. Add the handful of arugula and the teaspoon of capers. Toss everything gently together, so the ingredients blend well but the mackerel pieces stay intact.
Finish the salad by drizzling it with a good splash of olive oil. Toss everything one last time gently. This way the olive oil distributes evenly over the arugula, fennel and fish, which serves as a light dressing.
Divide the mackerel salad onto a plate or a shallow serving dish. Make sure the fish pieces and capers are nicely distributed over the whole dish. Serve immediately.