1/4 to 1/2cupbuttermilk (start with 1/4 cup, add as needed)
Instructions
Add the mayonnaise, sour cream, garlic, green onion, salt, lemon juice, parsley, black pepper, and cayenne (if using) to a bowl or container suitable for an immersion blender, or add to a regular blender.
Blend until smooth and fully combined.
Add 1/4 cup buttermilk and blend again, then add more as needed until the dressing reaches your preferred consistency.
Taste and adjust seasoning as needed.
Cover and refrigerate for at least 30 minutes before serving to allow the flavors to develop.