Combine the mustard and vinegar in a glass or stainless steel bowl and whisk them together briefly.
Place the mustard-vinegar mixture along with the oil and seasonings in a blender and mix for about 10 seconds or until fully combined.
Transfer to a glass bowl and let stand for 30 minutes to let the flavors meld. Give the dressing a good whisk immediately before serving.
Makes 1 cup of dressing.