In a large pot, heat the olive oil over medium heat. Add the chopped onion and sauté for about 5 minutes until translucent.
Stir in the minced garlic and chopped carrots. Cook for another 3-4 minutes, stirring occasionally, until the carrots start to soften.
Sprinkle the ground cumin and smoked paprika over the vegetables. Stir well to combine and let cook for another minute to toast the spices.
Add the diced tomatoes and vegetable broth to the pot. Bring the mixture to a boil and then reduce to a simmer. Let it cook for 20-25 minutes, or until the carrots are tender.
Using an immersion blender, purée the soup until it's smooth and creamy. If you don’t have an immersion blender, carefully transfer the soup in batches to a countertop blender.
Stir in the lemon juice, and season the soup with salt and pepper to taste. Let it simmer for an additional 5 minutes.
Ladle the soup into bowls and garnish with fresh basil leaves. Add croutons on top for extra texture, if desired.