In a large pot over medium heat, cook the chopped bacon until crispy. Remove the bacon with a slotted spoon and set aside, leaving the drippings in the pot.
Add the diced onion, carrots, and celery to the pot. Sauté for about 5-7 minutes until the vegetables are softened.
Stir in the minced garlic, thyme, and smoked paprika. Cook for an additional 1-2 minutes until fragrant.
Add the rinsed lentils, broth, bay leaf, and the reserved bacon to the pot. Bring to a boil, then reduce heat to low and simmer for about 30-35 minutes, or until the lentils are tender.
If using, stir in the chopped kale or spinach and cook for another 5 minutes until wilted.
Remove the bay leaf, season with salt and pepper to taste, and stir in the lemon juice before serving.