In a large bowl of water soak the lentils for approximately 1-2 hours, then drain and rinse.
In a large pot add the oil, chopped onion, carrots and celery, cook for 2 minutes stirring often then add the garlic and cook until onion is transparent.
Then add the lentils, broth, tomatoes and spices. Stir to combine, then bring to a boil, cover and simmer for 30 minutes. Add the potatoes and continue to cook for 30 minutes, in the last 5 minutes remove the bay leaf and add the spinach, stir to combine. At this point you can remove ⅓-½ the soup and blend until smooth* then add back into the pot. Heat through and serve.
*I removed about 3 ladles full (1 -1 ½ cups), I left mine not completely smooth, a little coarse.
Notes
If you use store bought broth, it is usually quite salty so best not to add any salt until the soup is almost cooked. Once it is almost cooked then taste for salt. Add if needed.The brown and green lentils will retain their shape well, whereas the, split red and yellow lentils will disintegrate, therefore, a lot of people prefer these to green and brown in soups and stews. Personally I prefer the brown which is what I usually use. Although if you are pureeing soup red and yellow would be best to use.Lentil Soup should be stored in an airtight container in the fridge. It can be re-heated on the stove or microwave. It will last up to three to four days in the fridge. It freezes very well too, freeze in a freezer safe container. The soup will last up to six months in the freezer.