Preheat oven to 400°. On a large rimmed baking sheet, toss sweet potatoes and red onion in oil then season with salt and pepper.
Distribute them evenly on sheet in a single layer. Bake until tender, about 20 minutes. Let cool for 10 minutes then transfer to a large bowl.
Meanwhile, make dressing: In a small bowl or in a medium liquid measuring cup, whisk together vinegar, mustard, honey, and spices. Gradually pour in oil, whisking constantly until emulsified. Season with salt and pepper.
Toss sweet potatoes with dressing, cranberries, feta, and parsley. Serve warm or at room temperature.