Put potatoes in a pot of cold water. Bring to a boil and cook until tender.
Set a steamer or colander on top of the pot of potatoes, and steam asparagus until just cooked, but still bright green. Drain potatoes and asparagus separately. Set potatoes and asparagus aside.
In a 9×13 baking dish or a deep pie plate, layer the filo sheets, brushing each layer with melted butter and leaving a 1-inch overhang around the edge of the dish. Put a damp tea towel over and set aside.
In a bowl, mash the potatoes with the cheese. Beat in the eggs, cream, and milk. Grate in the nutmeg, and season with salt and pepper. Spread this mashed potato mixture into the filo pastry, and scrunch up the sides of the filo. Line up the cooked asparagus on top of the filling. Brush the whole tart with any leftover butter.
Bake for 20-30 minutes, until egg is set and edges are browned. Let sit for 10 minutes before cutting.