Cook the root vegetables - Heat one tablespoon olive oil in a large pot or Dutch oven over medium heat. Add the onions and cook, stirring often, for 6-8 minutes or until the onions soften and turn translucent. Add the carrots and celery and mix well to combine with the onion. Continue to cook over medium heat for 10 minutes, stirring often.
Add the aromatics and spices. Reduce heat to medium-low and add the minced garlic. Cook, stirring continuously, for approximately 1 minute. Add the salt, dried thyme, basil, and sweet paprika, mixing well to combine.
Add lentils and broth. Rinse lentils under cool water. Add the diced tomatoes, vegetable broth, and lentils to the pot. Stir well to combine. Increase heat to high and bring to a simmer. Scoop any scum off the surface - discard.
Simmer. Cover and reduce heat to medium-low. Simmer for 40-60 minutes, or until lentils are soft. Stir occasionally.
Blend soup (optional). For a thicker, more incorporated soup, use an immersion blender to blend soup until desired consistency is reached. You may also transfer all (or part) of the cooked lentils and vegetables to a high-speed blender and blend until processed.
Stir in lemon and garnish. Stir in the lemon and lemon zest. Mix well to combine. Add additional salt and pepper, to taste. Serve in individual serving bowls garnished with fresh parsley, shredded parmesan, and diced tomatoes, if desired.