Brown the Italian sausage in a skillet over medium heat, breaking it into small pieces as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the diced onion, carrots, celery, and minced garlic to the slow cooker.
Stir in the rinsed lentils, diced tomatoes with their juices, chopped rosemary, bay leaf, salt, and black pepper.
Pour the chicken broth over the ingredients and stir gently to combine.
Cover and cook on low for 7 to 8 hours or on high for 4 hours, until the lentils are tender.
Remove the bay leaf before serving.