Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Toss together the sweet potatoes and beets on the baking sheet. Drizzle 3 tablespoons of the oil over top, sprinkle with salt and pepper and toss to coat. Spread into a single layer and roast until the potatoes are cooked through and the beets are tender, about 25 minutes. Remove from the oven and let cool.
Meanwhile, whisk together the mustard, Creole seasoning, onion powder and sugar in a large salad bowl. Whisk in the vinegar, then whisk in the remaining 2 tablespoons plus 1 teaspoon oil in a slow stream. Continue whisking until the dressing is emulsified.
Add the green and red cabbage and green onions to the bowl and toss to coat with the dressing. Add the roasted potatoes and beets and toss gently to combine. Add salt and pepper to taste.