Beat the eggs in a large bowl and add the oregano, basil and black pepper.Add the crumbled Feta, the spinach and the cooked onions and mix thoroughly.
Melt the margarine and butter (if using).Layer the baking sheet with phyllo dough, brushing each layer lightly with margarine. Keep the margarine warm so it will go on as thin as possible.Use half the sheets, (about 10 sheets), cut to size so that it fits your pan. Vary the sheets to cover the pan evenly.
Spread the filling evenly and continue with the phyllo sheets until all are used.Cut through the top layers to make either 12 or 24 pieces.
Bake at 375 for 45 minutes, or until the top is nicely browned.
Notes
Dinner serving sizeYou can make this a day ahead. Cover with plastic wrap, keep in the refrigerator and bake the following day. It may take an extra 10 minutes to bake if it's cold.