In a large saucepan, mix the 2 pounds sliced onions, prepared raisins, 4 tablespoons sugar or honey, 1 teaspoon pepper, 1 teaspoon cinnamon, 1/2 teaspoon ginger, 1/2 teaspoon turmeric, 1/4 teaspoon salt and 1/4 teaspoon crumbled saffron threads.
Add the 4 tablespoons butter and 1/2 cup water, cover, and bring to a simmer. Continue simmering for 30 minutes or longer over low heat, stirring occasionally, until the onions are very soft and golden. Add more water only if the liquids evaporate before the onions are cooked.
Once the onions are cooked and richly colored, reduce the liquids to a thick syrup. Remove from heat.