Combine onion,
celery, carrot, bacon, and garlic in a heavy, large Dutch oven.
Stir over medium-high heat 2 minutes.
Reduce heat to medium-low and cover and cook until veggies are tender,
about 7 minutes.
Uncover and add 7 cups broth, lentils, thyme and bay leaf
Bring to the boil.
Reduce heat and simmer uncovered until the lentils are tender, stirring
occasionally, about 45 minutes.
Discard thyme and bay leaf.
Transfer 1/2 of soup to a food processor, cool slightly.
Puree until smooth.
Return puree to Dutch oven.
(Can be prepared 1 day ahead. Cover and refrigerate.)
Bring soup to a simmer, thinning with more broth, if desired.
Season soup to taste with salt and pepper.