Drain the matjes fillets and then cut them into small pieces. Peel the jacket potatoes and cut into small cubes. Also cut the onions into cubes. Cut carrots, blanched or raw, into slices. Drain the beetroot, collect some liquid and also dice.
Layer the ingredients in a bowl in that order. Sprinkle salt and pepper between each layer. Then drizzle the salad with some beetroot juice. Then spread the mayonnaise over the salad. Let it steep for about 5 hours.