Heat olive oil in a large saucepan over medium-high heat, and add onion and carrots.
Sauté, stirring occasionally, for 8-10 minutes, until the onions and carrots begin to soften.
Add garlic and sauté for another minute.
Then add cumin and tomato paste and sauté for 90 seconds, stirring constantly, until tomato paste just begins to sizzle and brown.
Turn down the heat slightly. Add lentils, vegetable broth, 1 teaspoon salt, pepper, and water, and simmer for 25-30 minutes, until lentils are tender. Skim off the foam on the top of the soup as it is cooking. You may need to add more broth or water if your lentils are taking a long time to cook.
If you prefer a smoother texture for your soup, use an immersion blender to purée some of the soup. Or, put some of the soup into a blender and blend until smooth, then add back into soup pot. (Careful to let hot soup cool for a few minutes before blending).
If your soup has cooled, re-heat it over medium heat. Add spinach and stir until wilted.
Stir in 1 tablespoon lemon juice, taste, and add more lemon juice to taste. Add salt and pepper to taste.
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Notes
Some lentils take longer to cook than others. Check at 20 minutes, and then every 5 minutes after that until they are tender but still hold their shape.Make it your own: try adding more herbs like fresh thyme, fresh parsley, or a bay leaf to the vegetable mixture (before adding the broth). You could also add in chopped parsnips or a couple of celery stalks. A rind of parmesan cheese also adds great flavor (remove after soup is fully cooked). Some people like to replace the lemon juice with a little red wine vinegar or balsamic vinegar.