To make the dough: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. In a large mixing bowl, combine all of the dough ingredients. Mix and knead to make a smooth dough. Cover the mixing bowl and let the dough rise for 1 hour., After an hour, deflate the dough, divide it in half, and shape each half into a ball. Return to a greased bowl, cover, and refrigerate for 1 to 2 days., Prepare the oven: Place a rack in the upper third of your oven with a pizza stone on it. Preheat the oven at 500°F for 1 hour., To make the topping: Whisk together the créme fraîche and egg yolk. Season with salt, pepper, and a pinch of nutmeg. In a large skillet, cook the bacon halfway, until it releases some of its fat. Scoop out the bacon and drain on absorbent paper. Add the onions and cook over medium heat, stirring occasionally for 5 minutes, until they soften and become translucent. Remove from the heat, return the bacon to the pan, and set aside., To shape and assemble the pies: On a lightly floured surface, stretch the dough to a thickness of 3/8", leaving a 1/2"-thick rim around the outside edge. Sprinkle a pizza peel with semolina and slide it under the dough, or put the dough on a piece of parchment., Spread half of the cream mixture over the dough, then sprinkle with half of the bacon and onion mixture. Repeat the stretching and assembly process with the second round of dough., Slide the assembled pizza onto the hot stone and bake for 13 to 15 minutes, until the onions are browned and the edge of the dough is caramelized. Remove from the oven, sprinkle with chopped chives, and serve warm., Store any leftovers in the refrigerator, well wrapped, for several days.