In a large pot over medium heat add the oil, onion, garlic, carrots, cinnamon, ginger, crushed cumin seeds, pepper and pepper flakes. Sauté for 5 minutes or until the onion has softened. If the mixture seems dry and the spices start to stick to the pot, add up to ¼ cup of water to loosen things up a bit.
Add the liquidized canned tomatoes, chickpeas and 5 cups of water to the pot. Turn the heat up to medium-high and let the liquid come to a boil. Once the liquid is boiling, lower the heat to low, cover the pot with a lid and cook for 30 minutes.
After that time, add the puy lentils and cook, covered, for another 30minutes.
In the meantime, in a small bowl, combine the cornstarch with ¼ cup of water. Stir well so that the cornstarch dissolves completely.
At the very last minute of cooking, add the salt as well as the cornstarch mixture to the soup. Give it a good stir with a wooden spoon (it will thicken up a bit thanks to the cornstarch), have a taste and adjust the seasoning. Serve in large bowls, adding a splash of lemon juice to each individual serving.