Heat the oil in a large pot and add the veggies (garlic, onion, celery, and carrot). Cook over medium-high heat until golden brown, stirring occasionally.
Add all remaining ingredients except the salt and pepper. Stir.
Increase heat and bring to simmer. Cover with a lid, reduce heat to medium-low, and simmer for about 40 minutes or until the lentils are soft.
Finally, add the salt and pepper and stir.
To thicken the soup, use an immersion blender and do 3-4 quick pulses. You could also transfer 1 cup (250 ml) of the soup to a regular blender and blend, then transfer it back into the pot. This step is totally optional.