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 form.
Whisk the cream cheese and egg yolk until smooth and pale yellow.
Fold in the egg whites a little at a time until smooth and well combined.
Spoon the batter onto the baking sheet into two even circles.
Bake for 25 minutes until firm and lightly browned.
To complete the sandwich, layer the sliced meats and cheeses between the two bread circles.
Grease a skillet with cooking spray and heat over medium heat.
Add the sandwich and cook until browned underneath, then flip and cook until the cheese is just melted.
