Preheat the oven to 400 degrees F. Combine the peppers, raisins, parsley, cider vinegar, capers and 1/8 teaspoon each salt and pepper in a bowl.
Trim the base of the core from the cauliflower, and then cut the head into 1-inch-thick slices. Cut any florets that fall off into 1-inch pieces. Sprinkle 1/4 teaspoon each salt and pepper over both sides of each cauliflower slice. Heat the olive oil in a large nonstick ovenproof skillet over medium-high heat. Add the cauliflower steaks and florets and cook, turning the steaks once, until browned on both sides, about 5 minutes. Transfer the skillet to the oven.
Roast until a knife easily pierces through the steaks, 15 to 20 minutes. Spoon the relish over the steaks and serve.