Put all of the dressing ingredients in a jam jar, replace the lid and shake until well combined. Season with salt and pepper. Add more chilli if desired.
For the salad, rinse the black beans under cold running water and drain. Put in a large mixing bowl and pour over half of the dressing.
Add the remaining salad ingredients, apart from the watercress and avocado and stir well to combine.
To serve, divide the watercress among 4 serving plates. Spoon the salad on top, then cover with the avocado cubes. Drizzle a little more dressing over each portion.
