Add sliced carrots and leeks to a large Dutch oven (or soup pot) on medium-low heat with a tablespoon of cooking oil and cook until veggies begin to soften.

Add potatoes and tomatoes, stir a couple times to mix, then cook for an additional 2-3 minutes.

Add remaining ingredients and reduce heat to low. Cover and cook for 40 minutes until lentils have softened and about half of the liquid has been absorbed. (If you prefer a more stew-like texture, you can allow to cook for another 20 minutes).