Place the spinach and watercress in a large salad bowl.

Cut the ends off the oranges. Then place each orange cut side down on a cutting board and carefully slice the skin off. Once the skin is removed, cut the orange segments out from between the membranes with a small knife. Work over a bowl so that you can collect any juices. Squeeze all of the juice out of the remaining orange membranes- you should have about 2 tablespoons of juice.

To make the dressing, whisk two tablespoons of the collected orange juice with the vinegar, mustard and olive oil. Season the dressing with salt and pepper to taste.

Toss the greens with ¾ of the dressing. Arrange the avocado and orange slices on top of the greens and garnish with pomegranate seeds. Drizzle the remaining dressing over the top.