Combine ingredients using an immersion blender or food processor.
Add more salt and pepper to taste.
Notes
To make dairy-free ranch dressing: Omit the sour cream (or use a dairy-free brand) and use a dairy-free milk. Be sure to check the mayonnaise to be sure it is dairy-free. (Most mayos are, but some have a touch of milk.)To adjust the consistency: This ranch dressing is thick and creamy, more like a ranch dip, but you can add more milk to it to make it more of a salad dressing consistency if you want!