Heat a non-stick frying pan over a medium-high heat. Season the steaks and rub with the vegetable oil. Cook for 3-4 minutes per side, or until cooked to your liking. Transfer to a plate and leave to rest for 5 minutes.
Meanwhile, place the lentils in a mixing bowl, add the vinegar, oil, apple, chives, parsley and chilli. Season and stir well.
Scatter the radishes and watercress onto 2 plates and spoon over the lentils. Thinly slice the steak and place on top.