Cut cauliflower into florets and place in a large roasting pan. In a small bowl, whisk together oil, vinegar, sumac, pepper, and salt. Pour over cauliflower and stir so each piece is evenly coated. Cover with foil and roast at 400 degrees for 20-30 minutes or until cauliflower is soft. For a fancier presentation, top with fresh chopped parsley before serving.