Preheat oven to 500F (260C)
Heat butter in a frying pan. Add onions and brown sugar, stirring to combine. Cook over low heat and stir frequently. Cook until the onions are golden and very buttery soft- they should squish easily when poked with a spatula. This should take around 30 minutes.
Heat cast iron skillet on medium heat for a couple minutes
Add enough olive oil to the bottom of the skillet to completely cover it, about 2 tbsp.
Place the dough in the skillet, cook a few minutes until you will see it start to bubble up. Use a spatula to check the bottom of the dough, it should be lightly golden brown in color.
First add the caramelized onion to the dough, then top with mozzarella and some torn apart pieces of prosciutto. Add toppings all the way to the edges of the dough.
Bake in the oven until the cheese is completely melted, about 10 minutes.
Switch oven to broil on low until the cheese on top starts to crisp and brown. This should take around 2-3 minutes.
Remove cast iron pizza from the oven and let cool for several minutes.
Add more torn prosciutto and arugula to the pizza.
Drizzle with balsamic glaze
Cut and enjoy!