Pre-heat oven to 190 °C/375 °F/Gas 5. Cook the lentils in a small saucepan with the water until they form a stiff paste (approx 10-15 mins). Remove from the heat and add the cheese, onion and tomato puree. Season. Mix the egg into the mixture, using just enough to bind the mixture together. Grease a one-person casserole dish and press the mixture into it. Bake for 30-35 minutes until golden brown and firm to the touch. Serve with peas and boiled potatoes