In a large pot, heat the olive oil over medium heat. Add the diced bacon and cook until crispy, about 5-7 minutes. Remove the bacon with a slotted spoon and set aside, leaving the drippings in the pot.
Add the chopped onion, carrots, and celery to the pot. Sauté for about 5 minutes, or until the vegetables are softened.
Stir in the minced garlic, thyme, and bay leaf, cooking for an additional minute until fragrant.
Add the rinsed lentils, chicken broth, salt, and pepper. Bring the mixture to a boil, then reduce the heat to low and cover. Simmer for about 30-35 minutes, or until the lentils are tender.
Remove the bay leaf and stir in the chopped kale or spinach and lemon juice. Cook for an additional 5 minutes until the greens are wilted.
Serve the soup hot, garnished with the reserved bacon.