Sauté the sausage: In a large cast iron skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the sliced smoked Andouille sausage and cook for 3-4 minutes until lightly browned.

Transfer to slow cooker: Carefully transfer the browned sausage to your slow cooker. Add minced garlic, diced onion, carrots, and celery to the skillet and sauté for 2-3 minutes until aromatic. Transfer these into the slow cooker with Great Northern beans, dried oregano, and bay leaves, seasoning generously.

Add liquids: Pour in 4 cups of chicken broth and 2 cups of water to the slow cooker. Stir everything well to combine thoroughly.

Slow cook the soup: Cover the slow cooker with its lid and set it to cook on low for 7-8 hours or on high for 3-4 hours.

Add spinach before serving: About 10 minutes before serving, stir in the 3 cups of baby spinach and allow it to wilt.

Serve and enjoy: Once the spinach is wilted, ladle the soup into bowls and sprinkle with shaved Parmesan. Pair with crusty bread.