Heat the olive oil in a large, heavy saucepan. Add the onion, carrot and celery and cook over moderate heat until tender and slightly golden, about 8 minutes. Stir in the cumin and cinnamon and cook for 1 minute. Add the lentils and broth and bring to a boil. Reduce the heat, cover and simmer until the lentils are tender, about 20 minutes. Stir in the chard and simmer until tender. Add the lemon juice, season with salt and pepper and serve.