Preheat the oven to 400°. In a large skillet, heat the oil with the onion, garlic, cumin seeds, turmeric and cayenne. Cook over moderate heat, stirring frequently, until the onion begins to brown, about 5 minutes. Add the mashed potatoes, cabbage, tomato and cashews and cook, stirring, for 2 minutes. Cover and cook over low heat, stirring occasionally, until the vegetables are very tender, 10 to 15 minutes. Stir in the cilantro. Remove from the heat and stir in the cheese. Season generously with salt and black pepper.