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 to the pot. Bring to a boil, then reduce the heat to low and let simmer for about 30-35 minutes, or until the lentils are tender.
If using, stir in the chopped kale and cooked bacon during the last 5 minutes of cooking.
Remove the bay leaf before serving. Finish with a splash of lemon juice for brightness.