Heat oven to 200C/180C fan/gas 6. Place the unpeeled beetroots on a baking tray and roast for 35-50 mins, depending on their size. Give them a gentle squeeze after 35 mins – if they feel tender and are a little shrivelled, they are done; if not, continue cooking. Remove from the oven and set aside to cool.
Using a small sharp knife, carefully peel the beetroots, then cut into wedges (wear plastic gloves to prevent them staining your hands). Mix the dressing ingredients in a small bowl. Put the lentils, spring onions, apple, radicchio and mackerel in a large bowl, add half the dressing and toss everything together. Pile the ingredients onto a serving platter, layering with the beetroots as you do. Serve with the remaining dressing on the side.
