Simmer the lentils in a pan of water for 20 minutes. Peel the tomatoes after plunging them into boiling water for a few seconds. Chop roughly and place in a mixing bowl. Grate the courgettes and shred the sorrel; add to the tomatoes along with the olive oil, garlic, seasoning and lemon juice. Mix well. Drain the lentils, add to the other vegetables, mix well and serve warm or cold.