Put the onions, olive oil, and sugar into a skillet over medium heat and cook, stirring frequently, until the onions are wilted and brown, about 15 to 20 minutes. Reduce the heat if the onions are browning too quickly. Set aside to cool a bit.
Heat the oven to 450 degrees F.
Spread half the tomato sauce onto 1 of the crusts. Top with half the caramelized onions and half the mozzarella. Repeat with the other crust. Bake the pies until the mozzarella is browned and bubbling, about 10 to 15 minutes. As soon as the pies come out of the oven, sprinkle over the gorgonzola. Slice and serve.