Ether steam cauliflower or boil it in the salted water for about 2 minutes. Drain, transfer to a bowl or a jar.

Whisk together vinegar, olive oil, finely chopped red onions, and mustard. until emulsified. Season with salt and pepper. Drizzle vinaigrette over warm cauliflower, and add capers and parsley. Stir to combine.

Cover, and refrigerate overnight or up to 1 day. Serve chilled or at room temperature.