1. In a pot of cold water, plunge the beets. Bring to a boil. Cover and cook for about 35 minutes. Pierce the tip of a knife to the heart to check the cooking.
2. Drain the beets and peel. Cut the beets into sticks.
3. Drain the mackerel and separate the flesh into pieces. Reserve.
4. In a small bowl, mix the lemon juice and oil. Season generously with salt and pepper.
5. Arrange all the ingredients on 4 plates and drizzle with the lemony oil.
6. Refrigerate for 10 minutes before serving.