Cook the rice according to packet instructions, then tip into a large bowl and spread around to cool.
Meanwhile, bring a small saucepan of water to the boil, and add the asparagus. Cook for 3 minutes until just turning tender. Drain and add to the bowl with the rice.
Add all the remaining ingredients, and toss together until everything is fully coated with the dressing. Taste and season with a little salt if required.
Divide between two plates and serve immediately. (NB. If you are preparing this in advance, eg, to take to work in a lunchbox, leave the lemon juice and dressing off until you are about to eat it, or else the watercress will turn soggy and unpleasant).