Brown the sausage rounds in a skillet over medium heat until lightly golden on both sides. Drain any excess fat and transfer the sausage to the slow cooker.
Add the drained white beans, chicken broth, diced onion, minced garlic, thyme, salt, and black pepper to the slow cooker. Stir to combine everything evenly.
Cover and cook on low for 6 to 7 hours until the flavors have melded and the soup is hot throughout.
Stir in the fresh spinach during the last 10 minutes of cooking so it wilts but stays vibrant.
Taste and adjust salt or pepper if needed, then serve warm.