Preheat oven to 400°F. Brush a heavy large rimmed baking sheet with olive oil. Stir 3 tablespoons olive oil, coriander, cumin and cayenne to blend in a large bowl. Add the cauliflower and garlic and toss to coat. Spread the cauliflower out into a single layer on the prepared baking sheet. Roast until the cauliflower begins to brown at the edges, about 12 minutes. Stir and continue roasting until browned and tender, about 12 minutes longer. Transfer the cauliflower to a serving dish. Set the baking sheet over medium heat. Combine the raisins, water and vinegar on the baking sheet and boil, stirring up the browned bits from the bottom of the baking sheet until the liquid is reduced to about 2 tablespoons, about 1 minute. Transfer the contents of the baking sheet to the cauliflower. Add the parsley and walnuts and toss to combine.