Bring a pot of water to a boil, add the scrubbed and quartered potatoes along with some salt and simmer until tender, about 15 minutes.
While potatoes are cooking, whisk together the diced red onion, parsley, dill, apple cider vinegar, olive oil, stone ground mustard, sugar, salt, and black pepper in a bowl.
When the potatoes are fork tender drain them well. Pour the dressing over the hot potatoes and toss to coat. Divide among small serving cups and serve immediately.
Notes
The number of cups your will get will depend on the size of the cups you use.