Add onions, flipping to get them covered in the butter, fry them till limp and slightly caramelized, flipping them regularly (about 8-10 minutes) Removed from heat and let cool a little
In a bowl, beat together egg, sour cream and salt
Stir in chiles then onions, then 2/3 of the cheese
Fill shell, bake at 350 for 20 minutes or until it barely jiggles when shook
Top with remaining cheese and bake just until melted 2-3 minutes more