Melt butter in a large 6 quart pot over medium heat. Add in chopped onion and cook and stir until the onion is soft and translucent
Add in the chopped cauliflower and pour in the chicken stock, bring the soup up to a boil and cover with a lid. Allow to cook until the cauliflower is soft, about 10 minutes, stirring occasionally.
Using an immersion blender, blend the soup until the soup is smooth with no more large pieces of cauliflower or onion.
Combine the milk and flour in a separate bowl until smooth. Pour into the soup and bring up to a boil, turn down the heat to a simmer and stir until the soup has thickened.
Season with the Worcestershire, salt and black pepper.
Turn off the heat and add in the cheese, stir until the cheese is melted
Notes
If you don't have an immersion blender, the soup can be transferred to a blender and then blended until smooth. Be sure to leave a little opening in the blender to allow steam to escape.