In a mixing bowl, whisk together sour cream, mayonnaise, 1/4 cup milk and ranch seasoning mix. Cover bowl and refrigerate.
Heat olive oil a large cast iron dutch oven over medium high heat. Add onion and saute about 4 minutes until golden, add garlic and saute 1 more minute. Add chicken broth and diced potatoes and bring mixture just to a boil, then reduce heat to medium, cover pot and cook 15 - 20 minutes until potatoes are very tender.
In a mixing bowl, whisk together 2 cups milk with 1/4 cup flour (whisk very well to break up flour) and pour mixture into soup. Increase temperature to medium high heat, and bring mixture just to a boil, stirring constantly (stirring constantly will help break down the potatoes a bit).
Once mixture reaches a boil, reduce heat to low, add ranch dressing mixture from refrigerator, 1 cup (4 oz) grated Cheddar cheese, grated Parmesan cheese and season with black pepper to taste.
Cook about 5 minutes, stirring frequently (note, if you want smaller potato chunks in the soup you can simply mash the soup with a potato masher to break them down a bit more). Serve warm, sprinkle each serving with remaining 1/2 cup cheddar cheese, chopped bacon, and chopped green onions.
*you can use low sodium chicken broth if you prefer, for this recipe I just used regular and didn't add any salt to the soup. If you use low sodium you may want to add additional salt to the soup.