Roughly chop 1 garlic clove, the carrot and celery and put the mixture in a saucepan with the lentils, cover with water and bring to the boil over a medium heat. Simmer for 20 minutes, then remove from the heat but do not drain. In the meantime, finely chop the remaining clove of garlic and sweat it in the olive oil in a heavy frying pan over a medium heat until soft. Add the sausages and brown in the pan, then add the tomatoes. Continue cooking for about 15 minutes until the tomatoes have broken down and the sauce has thickened. Add the lentils with their cooking liquid, and season with salt and black pepper. Simmer gently for a further 15 minutes. Serve hot.