Preheat oven to 375F. Heat a large skillet over medium heat and add the ground beef, breaking it up into small pieces using a spoon. Cook until browned, stirring occasionally.

Transfer the beef to a plate with a slotted spoon and set aside for later. Drain off most of the grease from the skillet, leaving enough to keep it greased.

Reheat the skillet over medium heat and add in the onion. Cook until soft and beginning to brown, stirring occasionally. Stir in the garlic and cook for an additional 1 minute.

Add the beef back to the skillet. Stir in the Worcestershire sauce and cook until most of the sauce has evaporated. Stir in the salt and pepper. Remove from the heat. Taste and adjust the seasonings if necessary.

Bring about 2 inches of water to a simmer over high heat in the lower part of a double boiler. Once simmering, turn the heat to low.

In the top part of the double boiler (not over the heat), whisk together the almond flour, xanthan gum, and baking powder. Stir in the egg — the mixture should be very thick. Stir in the mozzarella cheese.

Place the top part of the double boiler over the bottom part with the simmering water. Heat the mixture, stirring frequently, until the cheese begins to melt and the mixture forms a dough. Use a silicone mitt to avoid being burned by steam.

Transfer the mozzarella dough to a piece of parchment paper. Knead it several times to incorporate any stray almond flour into the dough. Pat the dough into an oval shape. Cover with a second piece of parchment and roll out into an oblong shape. If the dough begins to wrinkle, flip it over to straighten out both pieces of parchment paper.

Spread the ground beef mixture lengthwise along the middle third of the dough, leaving about one-third of the dough on both sides. Squirt the mustard over the ground beef. Layer the red onions and sliced pickles over the mustard. Top with the cheddar cheese.

Fold the sides of the dough over the filling and seal together, crimping the edges if desired. Pinch the ends of the pastry together and decorate with a fork if desired. Using a sharp knife, pierce the dough in several places to create holes for the steam to release during baking.

Carefully slide the loaf onto a baking sheet and place it in the oven. Bake for 25–30 minutes or until it is golden brown. Let cool for 5 minutes on the baking sheet before transferring to a cutting board.

Slice crosswise and serve hot.