Combine the cauliflower, bottled clam juice, and drained reserved clam juice in a large pot and bring to a boil, reduce heat to a simmer and cook for about 10 minutes, or until the cauliflower is completely tender. Blend until completely smooth using an immersion blender or standard blender. Add the diced rutabaga and simmer on low, covered until tender, another 15-20 minutes.

Meanwhile, heat the bacon grease in a large non-stick pan over medium-high heat. Add the carrots, celery, and onion. Cook until the onion is soft and translucent, about 5 minutes. Add the mushrooms and cook until they release their liquid, another 5-10 minutes. Add the bay leaves and garlic and cook until fragrant, about 1 minute more. Add all of the softened vegetables to the cauliflower mixture along with the and Old Bay seasoning, thyme and celery salt. Simmer for about 10 minutes, or until the rutabaga is tender when tested with a fork. Adjust seasoning to taste with salt, and pepper, and more Old Bay seasoning, if desired. Stir in vinegar, clams, and parsley off heat just before serving.