Brush the potatoes and green onions with olive oil. Roast the potatoes until tender. Roast the green onions until softened. When cool enough to handle, cut the potatoes into 1-inch cubes and finely chop the green onions.
In a large bowl, whisk together the remaining 1/2 cup olive oil, the mustard, vinegars, and honey, and season with salt and pepper. Add the potatoes, green onions and parsley and mix until the potatoes are coated with the dressing. Season to taste with additional salt and pepper.