Spaghetti with Mussels and Watercress

• 2 lb / 900 g fresh mussels
• 14 oz / 395

Scrub the mussels, knock off any barnacles, and tug off the ‘beards’. Rinse the mussels thoroughly and discard any that fail to close when tapped. Put them in a saucepan, cover tightly and steam over a high heat for 2–3 minutes. Drain the mussels in a colander set over a bowl to collect the cooking juices. Discard any that fail to open. When cool enough to handle, shell the mussels and put them