Peel sweet potatoes and cut into ¾ inch pieces. Fill a Dutch oven with an inch of water and bring to a boil over high heat. Reduce heat to medium and lower a steamer basket into the pot. Add sweet potatoes to the basket. Cover and cook until sweet potatoes are nearly tender, about 15 minutes.

Add bell pepper pieces to the steamer basket and cook for an additional 2-4 minutes with the sweet potatoes until all vegetables are tender. Transfer vegetables to a large bowl.

Whisk vinegar and mustard with ½ teaspoon salt, ¼ teaspoon pepper and a pinch of cayenne in a small bowl. Drizzle half this mixture over the vegetables and toss gently to combine. Refrigerate at least 45 minutes to cool the vegetables.

Whisking constantly, drizzle the olive oil into the remaining vinegar-mustard mixture until fully incorporated. Pour this over the vegetables and stir gently. Stir in scallions and parsley and season with salt and pepper to taste.