Measure Greek yogurt, crumbled feta, dill leaves, apple cider vinegar, Worcestershire sauce, garlic powder, salt, onion powder, and black pepper into a blender or food processor. Blend/process until smooth and creamy. Add 1 tablespoon milk and blend again. Check consistency and, if a thinner dressing is desired, continue adding 1 tablespoon milk at a time -- blending in between -- until preferred consistency is reached.
Adjust seasonings to taste, transfer dressing to a bowl or jar, tightly cover, and refrigerate for a few hours before serving (which allows time for the flavors to blend). If the consistency of the dressing is too thick after chilling, thin it out with a splash of additional milk.