Put a large, generous pot of unsalted water on to start boiling.
Meanwhile, peel all the potatoes, sweet and regular, and cut into large chunks. When the water is boiling, add them to the water, and cook until tender -- about 20 minutes.
While they are cooking, grate the cheese and set aside.
Drain the potatoes, and mash them. Then add the butter, parmesan and milk. Season to taste with nutmeg, and serve.
Notes
2 oz (50 g) of grated Parmesan = ½ cup.Add more milk or butter if you please.