In an 8-quart pan over medium-high heat, stir bacon until browned around edges, about 2 minutes. Add onions, carrots, celery, and garlic to pan and stir frequently until vegetables are very limp, about 10 minutes (lower heat to medium if necessary to prevent scorching).
Add lentils, parsley, thyme, salt, pepper, bay leaves, and 9 cups water; bring to a boil. Reduce heat to maintain a simmer, cover, and cook, stirring occasionally, until lentils are very tender to bite, about 20 minutes. Add tomatoes, including juices, and 1 cup water; simmer uncovered for 10 minutes longer.
Ladle soup into bowls and serve with lemon wedges for squeezing over individual servings.