To Make The Potatoes: Move the oven rack to the bottom of the oven and then preheat oven to 450. In a large bowl, combine potatoes, olive oil, garlic powder and onion powder. Toss to coat and then spread out across a foil lined baking sheet. Bake for 25 minutes. Whisk together the vinaigrette while the potatoes are roasting.
In a large bowl, combine the cooked potatoes, tomatoes and dressing. Toss gently to thoroughly coat. Add the scallions and toss once more. Serve immediately or refrigerate until ready to eat. Bring to room temperature again prior to serving. Enjoy!
To Make The Dressing: Combine the mustard and vinegar in a medium size bowl and whisk to combine. Add the rest of the ingredients and whisk until thoroughly combined. Let the dressing stand for at least 30 minutes to meld the flavors. Whisk well or pour into a jar and shake well before serving.