Preheat oven to 400°F. On a large baking sheet, toss sweet potatoes and onion with 2 Tbsp. oil; season with salt and pepper. Arrange in a single layer.
Bake until tender, about 20 minutes. Cool for 10 minutes, then transfer to a large bowl.
Meanwhile, whisk together vinegar, mustard, honey, cumin, and paprika. Gradually pour in remaining ¼ cup oil, whisking constantly, until emulsified; season with salt and pepper.
Add dressing, cranberries, feta, and parsley to potato mixture and toss to combine. Serve warm or at room temperature.