In a large skillet, cook ground beef and onion until beef is browned and onion is tender; drain. Stir in the salt, oregano, garlic powder and pepper. In a large bowl, whisk the eggs, milk and mayonnaise; stir into meat mixture. Fold in cheeses.
Pour into pastry shell. Bake, uncovered, at 350° or until a knife inserted in the center comes out clean. Let stand 5 minutes before cutting.