Fry bacon in a large frying pan over a medium high heat until crispy. Keep the bacon fat in the frying pan, and set bacon aside (I put my bacon onto a few sheets of paper towel to soak up some of the fat).
Add onion to the bacon fat and cook over a medium heat, stiring until golden (about 8 minutes).
Whisk the cooked bacon, eggs, egg whites, milk, salt, pepper, nutmeg and 1/4 cup of the cheese in a medium bowl until combined.
Evenly scatter the onion onto the bottom of the pie shell. Pour the egg mixture on top and sprinkle with the remaining 1/4 cup cheese. Bake until the top is golden and the centre is no longer watery to touch, about 35-40 minutes.
I often make four at a time and freeze three after cooling and pre-slicing them.