Scrub potatoes.
Do not peel.
Cook in enough water to cover potatoes and the green beans, which will be added eventually.
Cook potatoes about 20 minutes, until tender.
Meanwhile, wash green beans and cut off tips.
When potatoes have cooked about 15 minutes, add green beans to pot and cook until green beans are done, 5 to 7 minutes.
They should be crisp tender.
If they are done before potatoes, remove green beans and continue cooking potatoes.
Drain.
While vegetables are cooking, combine vinegar, oil and mustard in a serving bowl large enough to hold vegetables.
Whisk the dressing to blend.
Cut scallions into thin rings.
Cut dill into small pieces.
Add scallions and dill to dressing.
When vegetables are cooked, add to bowl, cutting beans in half and potatoes into quarters.
Mix thoroughly with dressing and season with pepper.
Serve warm.