Preheat oven to 475 degrees F. If you have a pizza stone, preheat it with the oven.
Place dough on a cornmeal-dusted pizza paddle (if you're using a baking stone), or a lightly-oiled and cornmeal-dusted baking pan.
Lightly brush dough with olive oil. Layer with onions, Fontina, Gorgonzola, figs, and prosciutto.
Transfer the pizza to the oven and bake either directly on the stone or the pan for 8-15 mins (pan will take longer than stone), until the crust is golden and crisp, the cheeses are melted, and the figs are jammy.
Remove the pizza from the oven, let rest for a minute or two, and sprinkle with the baby arugula leaves. Season with a few grinds of black pepper. If desired, drizzle with balsamic reduction. Slice and serve.
*To make a balsamic reduction, heat 1 c. of balsamic vinegar in a pot on the stove until bubbly. Reduce heat and simmer for 10-15 mins, stirring occasionally, until the vinegar is thickened and coats the back of a spoon. The reduction will thicken as it cools.(This will make more drizzle than you need for the pizza. Store any extra, tightly covered, in the refrigerator, for up to a few weeks.)