In a skillet melt butter and cook onion for about 3 minutes or until softened. Remove skillet from heat and stir in spinach, feta cheese and egg. Season with salt and pepper. Preheat oven to 375 degrees. Lay out one layer of phyllo dough and brush with butter. Top with another layer of dough and repeat with butter and a third layer of dough. With a sharp knife cut rectangle into 3 evenly sized pieces. Place a heaping tablespoon of the spinach mixture in lower left corner of one of the pieces and fold up as you would a flag--folding the lower corner up to form a triangle and continuing to fold up in the shape of a triangle. Place triangles on a baking sheet, at this point they can be frozen and then stored in an airtight container. Bake for 15 minutes or until golden brown and puffed.