Mix dry mustard and cold water in a small bowl to form a paste. Let sit for 10 minutes.
In a medium mixing bowl, whisk together mayonnaise, buttermilk, and sour cream until smooth.
Stir in mustard paste, vinegar, Worcestershire sauce, garlic, scallions, and all herbs and seasonings.
Transfer dressing to an airtight container and refrigerate for at least 2 hours to let the flavors develop.
Serve chilled with salads, sandwiches, or as a dip. Enjoy!
Store in an airtight container in the fridge for up to 7 days. Do not freeze.