Make the sumac-pickled onions: Combine red onion, wine vinegar, sumac and ¼ teaspoon kosher salt in a small dish and set aside while you prepare the other ingredients, or ideally at least 15 minutes.
Combine all salad ingredients, including red onions and their pickling liquid, in a medium bowl and season to taste with salt and red pepper. Taste and adjust ingredients as desired.
This salad can be prepped ahead, but I’d keep the dressing off of it until at most an hour before serving so it doesn’t discolor the sprouts.
