Drain the lentils and place them in a large pan. Pour in plenty of cold water and add the onion and bay leaf. Bring to the boil, boil hard for 10 minutes, then lower the heat and simmer gently for 20 minutes.
Drain the lentils, discard the bay leaf and tip them into a bowl. Add salt and pepper to taste. Toss with the olive oil. Set aside to cool, then mix with the