Heat oven to 375°F. Saute onion in oil till it changes color to dark yellow. Add beef and brown it then add water, salt and pepper and let it cook till all the water has evaporated. If you decide to use nuts add them at this time. This is called asaag. Open the filo dough package and divide the sheets in half (they are usually sheets of 10x20 inch rectangles, you want to end up with 2 stacks of 10x10 inch sheets). In a greased 13x9 inch baking dish layer 2-3 sheets at a time and sprinkle them with a few drops of melted butter, and so on till you finish the first half of the sheets. Spread the meat and nut mixture on it and start doing the same thing with the other half of the sheets. Don't worry about spreading the butter on these top sheets. When you finish with all the dough cut the feteer in 2x1 inch squares with a sharp knife. Whisk together remaining melted butter, egg and milk and a pinch of salt. At this point, if you want, you can wrap the dish in plastic wrap and refrigerate till 1/2 hour before it is time to eat. Pour the egg mixture gently over the feteer and bake, uncovered, for 20 minutes or till the milk is absorbed and the feteer turns golden yellow.