Microwave sausage, onion, garlic, thyme, and pepper flakes in bowl, stirring occasionally, until onion is softened, about 5 minutes; transfer to slow cooker. Stir in broth, water, mushrooms, beans, bay leaves, and ¼ teaspoon salt. Cover and cook until beans are tender, 9 to 11 hours on low or 6 to 8 hours on high.
Step 2
Discard bay leaves. Stir in spinach and let sit until slightly wilted, about 5 minutes. Season with salt and pepper to taste. Serve.