Preheat the oven to 415°F. Line a baking sheet with non-stick foil and set aside.
Combine the cauliflower, carrots, celery, olive oil, salt and pepper in a zip top bag. Shake until well coated. Spread evenly onto the pan and bake for 20 minutes or until fork tender.
Meanwhile, heat a large stock pot over medium heat. Add the bacon and cook until crisp. Transfer to a plate lined with a paper towel.
Add butter to the pot along with the onions. Cook for 2 minutes, then stir in the garlic. Cook for an additional minute or until translucent. Slowly whisk in flour and cook for about 1 minute. Then slowly whisk in the chicken broth. Cook until it start to come to a light boil and has started to thicken, whisking occasionally. Add cooked vegetables and sour cream. Allow to simmer for 15 minutes.
Ladle into bowls and serve with chopped bacon, fresh parsley and parmesan cheese if desired.