Heat the oil in a large enameled cast-iron casserole. Add the onion, garlic, jalapeños and ginger and cook over moderate heat, stirring frequently, until the onion is lightly browned, about 6 minutes. Add the curry powder and cook, stirring, until fragrant and lightly toasted, about 2 minutes. Add the tomatoes, scraping up any bits stuck to the bottom of the casserole. Whisk in the water and peanut butter, season with salt and pepper and bring to a boil. Cook over moderately high heat for 15 minutes, stirring frequently.