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.
Whisk the oil, vinegar, and black pepper in a medium bowl to blend. Drizzle the vinaigrette 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.