Remove the pizza dough balls from the refrigerator to bring to room temperature while making the topping. Place a pizza stone in the center of the oven and preheat to 500 degrees.
Remove the casings from the sausage and brown until no pink remains, breaking up the sausage as it cooks. Remove from the heat and set aside.
Divide the caramelized onions, sausage, Gorgonzola cheese and mozzarella cheese into 6 portions. Finely chop the kale leaves.
One at a time, roll out each dough ball to a 12" circle. Transfer to a rimless cookie sheet sprinkled with cornmeal. Use your fingertips to dimple the dough to help make it crispy. Then top the pizzas with mozzarella, caramelized onion, sausage, Gorgonzola cheese and sprinkle with chopped kale and red pepper flakes if desired.
Transfer the pizzas to the hot oven. Jiggle the cookie sheet to make sure they are loose on top of the cornmeal. Then slide the pizzas onto the hot pizza stone. Bake for 7–8 minutes. Slide the cookie sheet back under the pizzas to remove from the oven. Transfer to a cutting board, sprinkle with red pepper flakes and grated Parmesan cheese if desired, slice and enjoy.