Soak the lentils in water for about 2 hours, then rinse well. Bring to a boil with fresh water in a saucepan and cook for about 1 hour (until tender). Drain, transfer to a bowl and allow to cool.
Mix in the onion, tomatoes and mint. Mix together olive oil and vinegar and mix into the salad. Season with salt and pepper.
Clean and wash the lettuce and arrange on plates. Spread the lentil salad on top and serve sprinkled with walnuts.