Warm oil in a large Dutch oven or soup pot over medium heat. Sauté onion until translucent (about 5 minutes). Add garlic, celery, carrots, tomatoes with their juices, seasoning, salt and pepper. Mix and cook for 2 minutes. Add broth and lentils. Increase heat and bring to a boil. Reduce the heat and gently simmer mostly covered for 25-30 minutes, until the lentils are tender but not mushy.

With a slotted spoon, transfer about half the vegetable-lentil mixture (not the liquid) to a bowl and set aside. Use an immersion or stand blender to puree the remaining vegetables, lentils and broth, giving the soup a thick, textured broth. Return the vegetable-lentil mixture you set aside to the soup and gently break up the tomatoes if needed. If you feel the soup is too thick, add some broth or water. Stir in spinach and cook until wilted. Finish with lemon juice. Taste and adjust seasoning to your liking.