Combine all ingredients except spinach in a 6- to 8-quart slow cooker. Cover and cook on LOW for 8 hours. OR combine all ingredients in an Instant Pot. Cover Instant Pot, and, using manual setting, set to 18 minutes on high pressure, making sure the pressure valve is sealed. When done, let the pressure slowly release.
Stir baby spinach into hot soup.
Optional: Let soup cool slightly, and transfer 4 cups to a blender. Process until smooth, and stir into remaining soup in pot before serving. OR use an immersion blender to partially blend soup.