Pour enough water (about 2 cups) into a large wide pot to come 1-inch up the sides. Cover and bring the water to a boil over high heat. Add the whole head of cauliflower to the boiling water. Reduce the heat to medium. Cover and steam until a skewer
inserts easily into the core
of the cauliflower, about 8 minutes. Transfer the cauliflower to the center of a 16 to 20-inch-diameter platter. Set aside to cool completely.
Arrange the giardiniera, corn, and roasted bell peppers in piles around the cauliflower
. Sprinkle the onions and olives over the vegetables. Sprinkle the capers over.
the oil, vinegar, and black pepper in a medium bowl to blend
over the vegetables, and serve. The salad can be prepared ahead. Cover the salad and refrigerate it overnight. Bring the salad to room temperature before serving.