Place the rinsed lentils, carrots, celery, onion, mushrooms, and garlic into the slow cooker.
Add the diced tomatoes with their juice, vegetable broth, thyme, bay leaves, salt, and black pepper.
Stir everything together until the ingredients are evenly distributed.
Cover and cook on low for 7 to 8 hours, until the lentils are tender and the broth has thickened.
Remove the bay leaves before serving.