In a 4-quart enameled cast-iron casserole, heat 2 tablespoons of the oil. Add 1 of the onions and cook over moderately high heat, stirring occasionally, until softened but not browned, about 4 minutes. Add the peppercorns and turmeric and cook, stirring, until fragrant, about 1 minute. Add the water, rice and salt and stir well. Bring to a boil, then cover tightly, reduce the heat to low and cook for 15 minutes.