Wash the potatoes and cook in boiling water until just tender. Drain the potatoes in a colander and run cold water over them. Cut the potatoes into quarters or halves as soon as they can be handled. Place the potatoes in a large mixing bowl.
Add the peas to the potatoes.
Chop the green onions and dill.
Place the white wine vinegar, Dijon mustard, salt, and pepper in a blender of a food processor and blend until combined (about 5 seconds).
With the motor of the food processor running, pour the oil in a slow, steady stream until incorporated.
Pour dressing over potatoes, add the onion-dill mixture and toss gently. Fold in the sour cream.
Let the potato salad stand for 30 minutes before serving.