Prepare the vinaigrette:

1. In a small bowl, whisk together all of the ingredients until emulsified. Season to taste with salt and pepper. Set aside.

Prepare the salad:

1. Bring a large pot of salted water to a boil.
2. Rinse and scrub the potatoes. Cut up into quarters (or eighths, if the potatoes are on the larger side).
3. Dump into the pot of boiling water and cook until just barely tender, about 5-6 minutes. Drain the potatoes and set aside.
4. Meanwhile, fire up your grill and preheat on high for 5 minutes or so, then drop the heat to medium-high.
5. Toss the green onions and peppers with 1 tbsp olive oil and season generously with salt and pepper. Arrange on the preheated grill and cook for 5-10 minutes, or until onions are lightly charred and peppers are tender-crisp, turning over every so often. Transfer to a platter.
6. Toss potatoes with remaining tbsp olive oil, and arrange on the grill. Cook for 10-15 minutes, or until quite soft and golden-brown all over, turning the potatoes occasionally and moving them around as needed to ensure all sides are evenly browned. Transfer to a large serving bowl.
7. Pour the prepared vinaigrette over the hot potatoes, and toss to coat evenly.
8. Trim the tops and bottoms off the onions, and brush off any charred or peeling skin from the peppers. Roughly chop, add to the salad and toss until evenly combined.
9. Cover bowl tightly with plastic wrap. Refrigerate the potato salad for at least 1 hour before serving, or up to 2 days.