In a large pot, sauté the onion and celery in the olive oil (2 tablespoons) over medium-high heat until tender and starting to brown (about 3 minutes).
Add garlic; sauté until fragrant (about 1 minute).
Add the chicken broth (6 cups), diced sweet potatoes, lentils, tomatoes, and spices. Bring to a boil, turn heat down, cover, and simmer on low for approximately 30 minutes (or until lentils and sweet potatoes are tender).
Using a potato masher, mash soup a few times in order to thicken it.
Stir in the spinach and allow to wilt. Stir in the vinegar and serve.
Notes
To make this in your slow cooker, omit the olive oil and add all ingredients except the spinach and vinegar. Cook on high for 4 hours or low for 8 hours. Then, pick up from step #4.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.