Put a pizza stone on the bottom rack in the oven and preheat to 500 degrees F for at least 30 minutes.
Heat a small skillet over medium heat, add the bacon, cook until the fat renders but is short of being crisp, about 5 minutes. Transfer with a slotted spoon to a paper towel-lined plate to drain.
Divide the dough into 2 equal portions; form each into a ball. Roll a portion into a 12-inch round on a floured surface (cover the other piece with a kitchen towel). Lightly dust a pizza peel with flour and put the dough on the peel, drizzle lightly with oil and sprinkle with salt and pepper. Slip the dough onto the preheated stone and cook until just set, about 4 minutes.
Using the peel, remove the dough from the oven and spread half of the mashed potatoes over the top, leaving about a 1-inch border around the side. Season with salt and pepper. Then sprinkle half of the cheese on top and top with half of the bacon, leek and garlic. Drizzle with a little olive oil. Return the pizza to the stone and cook until the crust is golden and the cheese melts, about 5 to 7 minutes more. Remove the pizza, top with half the grated Parmesan and scatter with some basil leaves and pepper flakes, if desired. Serve. Repeat with the remaining dough and toppings.
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