Place the cauliflower, broth and diced onion into a large crockpot.
Cook on low for 5-6 hours, or until the cauliflower is soft and cooked through.
In a medium size pot over low heat, melt the butter. Stir in the flour until it's combined; it will be all clumped together, this is ok.
Next, little by little, stir in the milk making sure that the milk is absorbed into the flour without making it lumpy. Eventually the mixture will be smooth.
When all of the milk has been added and the mixture is smooth, add in the cheese and stir until melted.
Move the cauliflower to one side of the pot so there is a soup surface to mix in. Alternatively, use a hand blender to puree the soup so that there are no cauliflower pieces, you can have a chunky or a smooth soup.
Whisk in your milk/cheese sauce into the crockpot.
Cook for another 15-20 minutes or so, stirring occasionally so there isn't a skin on the surface. Serve and enjoy!
Notes
Nutritional values may vary.To make this soup low carb, simply stir in ONLY the cheese at the end, omitting the flour. You can also then use 1/2 cup of whipping cream instead of the cup of milk at the end, and no flour. It works out great!