Place all ingredients except oil in blender container.
Blend until combined.
Slowly add oil while machine is running.
Blend until combined.
By Hand
Place the Dijon mustard in a small bowl.
Measure one of the vinegars. Add about 1 teaspoon vinegar, and mix thoroughly. Add another teaspoon vinegar and mix well. Continue to add a small amount of vinegar until the mustard is thinned out and will mix well with the other ingredients. *
Add salt and other vinegar & maple syrup.**
Slowly whisk in oil.
Notes
*You can try to just whisk all the ingredients together, but when I try this, sometimes the mustard clumps and it is hard to get it to mix in. That's why I suggest this method.
** You can easily add the maple syrup at the end if you want to try the dressing without it first. If it is too strong or too bitter, add the maple syrup. I am always amazed at how much the maple syrup mellows this dressing.