Place the broccoli, cream cheese, onions, garlic, chicken broth, salt, pepper, and red pepper flakes in a large slow cooker. Cover and cook on high for 2 hours or low for 4 hours.
Add the cheese and spinach and stir until the cheese is melted. Use an immersion blender to blend until smooth. You can also leave it a little chunkier, if you prefer.
Season to taste with additional salt and pepper before serving.
Video
Notes
Storage Information: Store this soup in a covered container in the fridge for up to 7 days.