Crisp the Bacon: Place the finely chopped bacon in a large pot over medium heat and cook, stirring frequently, until the fat has rendered and the bacon is crisp, about 4–5 minutes.
Soften the Vegetables: Add the olive oil, finely chopped onion, celery, diced carrots, and chopped garlic to the pot. Cook over medium heat, stirring occasionally, for about 5 minutes, until the onion becomes soft and translucent. Keep the vegetables from browning by reducing the heat if necessary. Build the Soup: Add the diced tomatoes with their juices, chicken broth, lentils, dried thyme, bay leaves, salt, and freshly ground black pepper. Stir to combine the ingredients thoroughly. Bring to a Boil: Increase the heat as needed and bring the soup to a boil.
Simmer Until Tender: Partially cover the pot, reduce the heat to low, and simmer until the lentils are tender, about 45–50 minutes when using French lentils. If using common brown or green lentils, begin checking for tenderness at around 30 minutes. Remove the Bay Leaves: Once the lentils are tender, carefully fish out both bay leaves and discard them.
Partially Purée the Soup: Use an immersion blender to purée the soup until the broth is slightly thickened or reaches your desired consistency. Avoid over-puréeing, as the soup can become too thick and lose the texture of the whole lentils. Blend Without an Immersion Blender: If you do not have an immersion blender, transfer about 2 cups of the soup to a blender and purée until smooth. Return the blended portion to the pot and stir it into the remaining soup. Garnish and Serve: Ladle the soup into serving bowls and garnish with chopped fresh parsley if desired. Serve while hot.
Adjust the Consistency: If the soup thickens as it sits, stir in a little water as needed to reach the desired consistency.
