Heat 1 tbsp olive oil in a medium-sized saucepan. Add the lentils, 1 tbsp dressing, ½ the watercress and 150ml of water, and cook for 7-8 mins or until the watercress has wilted and the lentils have broken down a little. Season.
Heat a griddle pan over a medium heat, drizzle the steaks with oil and cook on either side for 5 mins. Serve with the lentils and remaining watercress tossed in the rest of the dressing.