In a large skillet over medium-high heat, brown the sausage. When just browned, add the cloves garlic to the skillet and cook for 30 seconds. Drain the fat from the skillet and spoon into a large slow cooker.
Pour the broth and water into the slow cooker. Add the chopped onion, celery and potatoes. Stir in the thyme, basil, rosemary, oregano, and red pepper flakes (if your family likes a little heat). Season with salt and pepper and cook on low for 4 hours.
Before serving, add the evaporated milk and kale to the slow cooker. Stir to combine and allow the kale to wilt a bit and the milk to heat through.
Ladle into bowls and serve with grated mozzarella for garnish.