For the bread, preheat the oven to 300°F and line a baking sheet with parchment.
Beat the egg whites with the cream of tartar and salt until soft peaks
Whisk the cream cheese and egg yolk until smooth and pale
Fold in the egg whites a little at a time until smooth and well
Spoon the batter onto the baking sheet into two even
Bake for 25 minutes until firm and lightly
Cook the bacon in a skillet until crisp, then drain on a paper
Assemble the sandwich with the bacon, avocado, lettuce, and tomato.