Preheat the oven to 450°. In a small bowl, cover the saffron with 1 tablespoon of the hot water and set aside. On a rimmed baking sheet, toss the onion wedges with 1 tablespoon of the olive oil. In a large roasting pan, toss the cauliflower and sweet potato with 2 tablespoons of the olive oil. Season all of the vegetables with salt and pepper and roast for 10 minutes, adding the garlic to the onions in the last 3 minutes. Remove the onions and garlic from the oven. Add the beans, tomatoes and their juices, wine, cumin, crushed red pepper and saffron water to the cauliflower and sweet potatoes; roast for 10 minutes longer. Meanwhile, in a medium saucepan, bring the remaining 1 1/2 quarts of water to a boil. Whisk in the polenta; cook over moderate heat, whisking constantly, until thickened, about 5 minutes. Stir in the remaining 2 tablespoons of olive oil and 1/4 cup of Parmesan; season with salt and pepper. Spoon the polenta into 4 bowls. Stir the onions and garlic into the other vegetables, season with salt and pepper and ladle over the polenta. Serve, passing the remaining Parmesan at the table.