Preheat an oven to 450F and arrange the oven's rack to the center position. Divide the dough into two pieces and roll them out on a floured board to about twelve inches in diameter. Dist a baking sheet with the corn meal, and gently transfer the rolled dough onto the baking sheet. Drizzle the oil across the doughs, sprinkle them with the garlic, and gently rub the garlic into the dough. Using a spoon or rubber spatula, spread the labna across the doughs, then sprinkle them with the za'tar and salt. Then arrange the sliced onion evenly across the pizzas. Allow the pizzas the rest for about 20 minutes before baking. Bake the pizzas for 10-15 minutes, or until the crust is crisp and the topping is bubbling and slightly browned. Transfer the pizzas to a wire rack and allow to cool for a couple of minutes before slicing.