In a large skillet, heat the oil. Add the potatoes and cauliflower and cook over moderate heat, stirring, until golden in spots, 7 to 8 minutes. Stir in the garlic, ginger, cumin seeds, mustard seeds, coriander and curry leaves and cook until fragrant, 1 minute. Season with salt and pepper and add 1 1/2 cups of water. Cover and simmer over moderately low heat, stirring occasionally, until the vegetables are tender, 15 to 20 minutes. Stir in the tomato, peas and butter and cook for 2 minutes longer. Transfer the <em>aloo gobi</em> to a platter, garnish with the cilantro and serve.