In a bowl, whisk together the egg, shredded mozzarella cheese, almond flour, baking powder, salt, and pepper.
Preheat a waffle iron and lightly grease the surface.
Pour the mixture onto the waffle iron and spread it evenly.
Close the waffle iron and cook for 3-5 minutes, or until the chaffle is golden brown and cooked through.
Remove the chaffle from the waffle iron and let it cool slightly.
Meanwhile, cook the burger patty in a pan or on a grill until it reaches your desired level of doneness.
Assemble the chaffle burger by placing the lettuce leaves, sliced tomato, sliced red onion, and cooked burger patty on top of one chaffle.
Top with another chaffle and serve with mustard and mayo, if desired.
