Spray a Dutch oven with nonstick spray coating. Heat Dutch oven over medium heat. Brown half of the lamb. Remove lamb from Dutch oven. Repeat with remaining lamb and all of the onion. Return first half of lamb to Dutch oven.
Add water, undrained tomatoes, carrots, celery, salt, thyme, pepper, garlic, and bay leaf. Bring to boiling; reduce heat. Cover and simmer for 30 minutes.
Rinse lentils; add to lamb mixture. Bring to boiling; reduce heat. Cover and simmer for 30 minutes more or until lamb and lentils are tender. Remove bay leaf. Makes 6 servings.