Clean and bone the mackerel and marinate it in the oyster sauce, sea salt and coarsely ground black pepper.
Wash and peel the cucumber. Then slice it lengthways in half and remove the seeds. Slice the cucumber lengthways once more into thin strips.
Cover a clean work surface (or sushi mat) with transparent plastic wrap. Divide the smoked salmon over the width of the nori sheet. Carefully spread a layer of mackerel over the salmon followed by two strips of cucumber.
Roll up the ingredients to form a sushi roll. (Make sure the roll is rolled up tightly enough in the beginning). Repeat this procedure until the ingredients are evenly divided.
Place the sushi rolls in the fridge for at least 30 minutes, allowing them to set. (You can also do this one day in advance).
Meanwhile, make the pickled cauliflower as follows: bring a pan filled with water to a boil together with the turmeric and a pinch of salt. Wash the cauliflower. Break it into florets and blanch them until tender in the turmeric water. Rinse the cauliflower in a colander. Rinse the chili pepper and slice it into thin slices. Set the chili peppers and cauliflower aside. When the sushi rolls are thoroughly cooled, slice them into pieces measuring 2 centimetres thick.
Place two salmon-mackerel sushi on a plate and garnish them with the pickled cauliflower, shiso leaves, fresh chili pepper and a few optional dashes of wasabi mayonnaise.
Serve the sushi as a snack or as a side dish at an Asian-themed lunch