Fry the bacon in a large pot over medium heat, stirring frequently, until the fat is rendered and the bacon is crisp, 4-5 minutes. Add the olive oil, onions and celery and cook, stirring occasionally, until the onions are soft and translucent, about 5 minutes. Add the carrots and garlic, stirring constantly, and cook 1 minute more. Add the diced tomatoes (with their juices), chicken broth, lentils, thyme, bay leaves, salt and pepper and bring to a boil. Cover partially, reduce heat to low and simmer until the lentils are tender, 45-50 minutes (less for common lentils). Fish out bay leaves and discard.
Use an immersion blender to purée the soup until the broth is slightly thickened, or to desired consistency. (Do not purée too much or the soup will get too thick, and you'll lose the integrity of the lentils.) If you don't have an immersion blender, transfer about 2 cups of the soup to a blender and purée until smooth, then return the blended soup to the pot. Garnish with fresh chopped parsley if desired and serve. (Note: The soup may thicken as it sits; thin with a bit of water if necessary.)