Melt butter in a large soup pot over medium heat. Add the flour and cook, stirring, for about 1 minute. Add the chicken broth, milk, chopped cauliflower and 1/3 cup chopped green onion. Increase heat to medium-high heat and bring to a boil, then reduce heat to medium-low and simmer, stirring regularly, until cauliflower is tender. Using a potato masher or an immersion blender, break up the cauliflower pieces just slightly, being sure to leave some chunky pieces in the soup.
Stir in sour cream and 1/3 cup shredded cheddar cheese and allow to warm through for a few minutes. Taste soup and season well with salt and freshly ground pepper (Be generous with the salt. If it tastes bland at all, it needs more salt!)
Serve topped with additional shredded cheddar cheese, chopped green onion and crumbled cooked bacon.