Cook the lentils in a saucepan of boiling water for 30 minutes or until tender. Meanwhile segment the oranges or clementines, catching any juices in a bowl. Arrange the watercress, orange segments and smoked mackerel in serving bowls.
To make the dressing, mix together the horseradish, oil and seasoning and any caught orange juices. Drain the lentils, stir into the dressing and spoon over the salad and serve.
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