Preheat oven to 425 degrees. On a nonstick rimmed baking sheet, toss sweet potatoes with olive oil, coarse salt, and ground pepper; roast until fork-tender, about 35 minutes.
In a large bowl, whisk together fresh orange juice, oil, minced fresh ginger, and Dijon mustard.
Add scallions and potatoes; toss to coat with dressing. Season with salt and pepper, if desired. Serve warm, cold, or at room temperature.
To make ahead, prepare the recipe through step 2, and then cool completely before storing in an airtight container in the refrigerator for up to one day. Just before serving, proceed with step 3.