Preheat the oven to 350°F (175°C). Line a rimmed baking sheet with parchment paper.
In the bowl of a food processor fitted with the steel blade attachment, pulse together the almond flour, coconut flour, xanthan gum, baking powder, and 1 teaspoon salt.
In a small bowl, beat 3 of the eggs with the cold water.
With the food processor running, add the apple cider vinegar. Stop the processor, add the beaten eggs, and pulse until the mixture becomes a sticky dough ball, about 8 to 10 pulses.
Wrap the dough in plastic wrap and knead it through the plastic for 3 minutes. Refrigerate, wrapped, for 10 minutes.
Divide the dough into 8 equal (1/4 cup / 2 oz) portions. Roll each into a ball. Cover the balls with plastic wrap or under a damp towel.
Working one at a time, flatten the sticky ball onto a piece of parchment or waxed paper, cover with another piece of parchment, and flatten to a (4½ by 5½ -inch) oval. Spoon 1 teaspoon marinara sauce on one half of each oval, leaving a ½-inch border. Scatter 1/4 cup (1 ounce) of the cheese blend over the sauce. Top with 1/2 ounce (about 7 slices) of pepperoni, then fold the topping-free half of dough onto the topped half to create a half-moon-shaped pie. Dampen your index finger, use it to gently press the edges together, then crimp/seal the edges with the tines of a fork. Carefully place the hand pie on the prepared baking sheet. Repeat with remaining dough and fillings.
In a small bowl, beat the remaining egg with 1 tablespoon water, then brush the top of each pie with the egg wash. Sprinkle on the dried herbs and a bit of pepper.
Bake, rotating the pan once during baking, until the crust is golden brown, 22 to 25 minutes. Let cool on the pan for 5 minutes. Serve.