In a large pot, heat the olive oil over medium heat. Add the chopped onion, garlic, carrot, and celery. Sauté for about 5-7 minutes until the vegetables are softened and fragrant.
Add the ground cumin and smoked paprika, stirring to combine. Cook for an additional 1-2 minutes to release the spices' aroma.
Pour in the lentils and vegetable broth, bringing the mixture to a boil. Once boiling, reduce the heat to a simmer and cover the pot. Let it cook for about 20-25 minutes, or until the lentils are tender.
Stir in the drained butter beans and chopped spinach. Allow the soup to simmer for another 5-10 minutes, until the spinach is wilted and heated through.
Season the soup with salt, pepper, and the lemon juice. Adjust seasoning as necessary.
Serve the soup hot, garnished with fresh parsley for an added touch.