Place lentils in a large pot with enough water to cover by a few centimetres. Bring to the boil and cook until lentils are tender, about 15 minutes. Drain and rinse with cold water in a colander to cool. Let stand for a few minutes to drain well.
In a large bowl, whisk together the olive oil and lemon juice. Add the red capsicum, onion and green olives then toss to blend. Refrigerate for about 2 hours before serving to blend the flavours.