Brown the sausage in a skillet over medium heat.
Drain excess fat and add the sausage to the slow cooker.
Add the diced onions, carrots, celery, and garlic to the slow cooker.
Stir in the lentils, diced tomatoes, thyme, bay leaves, salt, and pepper.
Pour the broth over the mixture in the slow cooker.
Cover and cook on low for 6-8 hours or high for 3-4 hours.
In the last 30 minutes, stir in the spinach or kale (if using).
Taste and adjust seasoning before serving, removing bay leaves.