Fill a Dutch oven with 1 inch of water and bring to a boil over high heat. Reduce the heat to medium-low and carefully lower a steamer basket into the pot. Add the sweet potatoes to the basket, cover, and steam until the potatoes are nearly tender, about 15 minutes.

Add the bell pepper to the steamer basket, cover, and continue cooking until both the sweet potatoes and peppers are tender, about 2 to 4 minutes. Transfer the steamed vegetables to a large bowl.

Whisk the vinegar, mustard, salt, pepper, and cayenne together in a small bowl. Drizzle half of this vinegar mixture over the warm vegetables and toss gently to combine; refrigerate until the vegetables are completely cool, about 45 minutes.

Whisk the oil constantly while drizzling it into the remaining vinegar mixture until emulsified, then gently stir the finished dressing into the cooled vegetables. Stir in the scallions and parsley, season with additional salt and pepper to taste, and serve.