Slice or dice the avocado flesh and put it in a shallow bowl with the lemon juice.
Cut the mango in half on either side of the central stone, peel away the skin and slice or dice the flesh.
Make the dressing. Whisk together the oil, mustard, honey and vinegar. Season to taste with salt and pepper. Remove the avocado from the lemon juice and mix the juice into the dressing.
Arrange the watercress and beetroot on 4 plates or in a salad bowl and add the avocado and mango. Drizzle the dressing over the salad and stir to combine. Thinly slice the chicken and top the salad with the meat. Serve immediately.