In a medium saucepan, cover the potatoes with cold water, add salt and bring to a boil. Cover and simmer over moderate heat until the potatoes are just tender throughout, 10 to 15 minutes. Drain in a colander, rinse under cold running water and drain again; pat thoroughly dry.
When the potatoes are cool, cut 1/2 inch off the top of each; reserve the caps. With a melon baller, scoop the insides of the potatoes into a bowl, leaving a 1/4 -inch-thick shell. Coarsely mash the potatoes in the bowl; reserve the shells and caps.
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.
Using a teaspoon, stuff the potato shells with the filling and cover with the reserved caps. Set the potatoes in a baking dish and brush with the melted butter. Bake for about 20 minutes, or until lightly browned and heated through. Serve hot or warm.
The stuffed potatoes can be refrigerated overnight. Bring to room temperature before baking.