• Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.

Make The Fathead Dough:

• In a large microwave-safe bowl, microwave the mozzarella cheese in 30-second intervals until it becomes very melted and stretchy.
• Add the egg and incorporate it completely into the mixture.
• Add the almond flour and baking powder to the bowl. Mix with a wooden spoon until the ingredients are completely combined.
• Wet your hands slightly, then knead the dough until it's a uniform color with no obvious white or yellow splotches.
• Place the dough between two pieces of parchment paper and roll it into a thin and even rectangle. Set aside.

Make The Filling:

• Place the chopped deli meats and bacon in a medium-sized bowl. Sprinkle in the onion powder and cheddar cheese. Toss to combine.
• Add the egg and mustard. Stir gently until all the ingredients are fully combined.

Assemble And Bake The Sandwich Puff:

• Spread the meat mixture in an even layer on one half of the fathead dough.
• Fold the untopped half of the fathead dough overtop of the meat mixture. Fold the sides and ends up to meet the top crust, and pinch the edges together. You may need to dab the edges with a little water to help them stick together.
• Score the top crust with a knife to let steam escape. If desired, brush the top with melted butter.
• Bake until golden brown, about 12-15 minutes.
• Remove the puff sandwich from the oven and let it rest on the pan for 5-10 minutes.
• Slice into desired portions. Serve immediately.