Wash potatoes and pat dry. Using a sharp knife or mandoline slicer, cut potatoes into ¼ inch-thick round slices.Set aside.
Meanwhile, heat half the olive oil in a skillet over medium heat; add onions. Cook the onions over medium heat, stirring occasionally. During the next 15 minutes, they’ll first soften and become translucent, then gradually turn slightly golden. Let cook an additional 10 minutes, stirring occasionally to prevent burning, until onions are a rich, golden color. Remove from heat.
Preheat oven to 450 degrees F. Roll out pizza dough on a lightly floured surface and transfer to a parchment lined baking sheet. Brush dough lightly with olive oil. Place ½ cup of shredded mozzarella on the dough, then arrange potato slices on top. Season well with salt and pepper. Top with remaining cup of mozzarella, parmesan and half of the fresh rosemary.
Bake pizza for about 20 minutes, or until both the top and bottom of the crust is brown and the cheese is melted. Before serving, sprinkle pizza with remaining rosemary and freshly ground pepper, if desired.
Recipe by In Good Taste at http://ingoodtaste.kitchen/purple-potato-pizza-with-caramelized-onions-and-rosemary/