Whisk together flour and 1/4 cup chicken broth in a small bowl then set aside.
Saute bacon in a large soup pot over medium heat until crisp. Using a slotted spoon, transfer bacon to a paper towel-lined plate then remove all but 1 Tablespoon drippings from the pot. Add chopped onion (if using,) celery, and garlic to the pot then season with salt and pepper and saute until vegetables are tender, about 4-5 minutes.
Add cauliflower and onion powder (if using) to the pot then stir to combine. Add water then place a lid on top and steam cauliflower until tender, stirring a couple times, about 5-7 minutes. Add remaining chicken broth and milk then turn up heat and bring to a boil.
Slowly whisk in flour/chicken broth mixture while stirring, then turn down heat and simmer for 3-4 minutes, or until chowder has thickened. Turn off heat then stir in 2 cups cheddar cheese until smooth, then stir in half the cooked bacon. Taste and adjust salt, pepper, and/or hot sauce if necessary. Serve topped with remaining shredded cheese, cooked bacon, and green onions, if desired.
NOTE: Use a food processor's chopping and grating blades to make prepping veggies and cheese a breeze.