Add the Dijon mustard and white vinegar, blending until well combined.
Gradually pour in the oil while blending continuously until the mixture thickens. (You can substitute this step with ½ cup of mayonnaise for convenience.)
In a mixing bowl, combine the prepared mayonnaise base with sour cream, lemon juice, onion powder, garlic powder, parsley, dill, salt, and pepper. Whip until all ingredients are fully incorporated.
Slowly add the milk while stirring, adjusting the amount to achieve your desired dressing consistency.
Serve immediately or transfer to an airtight container. Store in the refrigerator and use within the best-by date of the dairy ingredients.
Notes
Use Room Temperature Ingredients: Room temperature eggs and mayonnaise blend more smoothly and result in a creamier texture.Chill Before Serving: Allow the dressing to chill in the refrigerator for 30 minutes to let the flavors meld together.Blend for Consistency: If the dressing looks lumpy, give it a quick blend to achieve a smooth and uniform texture.