In a large soup pot over medium-high heat, melt bacon fat until it glistens. Add half the meat in a single layer and let it cook undisturbed for 3 minutes, until browned. Use a spatula to flip meat over. Repeat, browning meat on all sides, then transfer to a plate and set aside. Add remaining half of meat and repeat browning process. Once all the meat is browned, return it all to the pot and lower heat to medium.
Add onion and celery to pot and sauté for 5 minutes, stirring frequently, until softened. Add garlic and sauté for 1 minute, stirring constantly, until fragrant.
Add 2 cups of stock and use a spatula to scrape browned bits from the bottom of the soup pot. Add remaining 6 cups stock and bring to a boil. Lower heat to a rolling simmer. Cover and simmer for 1 to 4 hours; additional time simply yields more tender meat.
After simmering, add cabbage and carrots to the pot. Bring to a boil. Cover, lower heat to a rolling simmer, and simmer for 10 minutes.
After 10 minutes, add all the remaining ingredients, except the spinach. Bring to a boil. Cover, lower heat to a rolling simmer, and simmer for 20 minutes.
Right before serving, stir fresh spinach into the simmering soup. Turn off the heat, wait 5 minutes, and serve.
Recipe excerpted from Back to Butter: A Traditional Foods Cookbook (Fair Winds Press, 2014) by Molly Chester and Sandy Schrecengost.