Cut the stem off the cauliflower and remove any green leaves. Cut in half, then cut each half in half, lengthwise, making 4 steaks. Drizzle both sides of the cauliflower steaks with olive oil and season with salt and pepper. Bake for 10 minutes, flip the cauliflower steaks over, and bake for another 15 minutes or until fork tender and golden brown.
While the cauliflower is baking, make the sauce. Combine the vinegar, honey, water, raisins, onion, and red pepper in a medium saucepan. Bring to a boil over high heat. Reduce the heat and let boil gently, uncovered, for 12-15 minutes or until most of the liquid has evaporated and is syrupy and the raisins and onions are tender.
Serve the cauliflower steaks topped with the sauce and garnish with the nuts and parsley.