Pour the chicken or veggie stock into a large soup pot with the chopped tomatoes and set it aside.
Prepare your onion, celery, and carrots, and sauté them over low to medium heat using the oil until the onions are nicely caramelized. This can take a little time, so be patient.
Add the celery and onion mix along with the potatoes to the soup pot and stir in the spices.
Bring to a boil and cook for 20 minutes over high heat.
Stir in the lentils and continue to cook, reducing the heat just slightly. Watch the pot as the lentils can burn on the bottom of the pot if you don't stir them often.