Make the pickled sumac onions: In a small bowl, toss the onions with the red wine vinegar, sumac and salt. Let the onions macerate for at least 30 minutes, stirring occasionally. Makes 1 cup. Make ahead: The pickled onions can be made, covered and chilled for up to 3 days.
In a separate large bowl, combine all of the tabbouleh ingredients and toss. Taste and adjust the seasoning with salt. Sprinkle with more pomegranate seeds and serve.