Instructions

1. At least one hour before and up to several days ahead, make the pie dough:  If the butter is fully frozen, remove from the freezer and let thaw briefly, 5 minutes or so.  In a medium bowl, whisk together the flour, Parmesan, and salt. Fill a separate small bowl or cup with ice and add ⅓ cup cold water.  
2. Using the largest holes on a box grater, grate the frozen butter into the bowl, then mix with a wooden spoon or your fingers until pea-sized crumbles form. Add ¼ cup of the ice water and stir just until a crumbly dough forms. If a fistful of the dough holds together when squeezed, the dough has enough moisture; if it doesn’t, drizzle more water, one tablespoon at a time, until the dough holds together. Gently gather and knead the dough together into a round, flat disk. If it’s unwieldy, pile the dough in the center of a piece of plastic wrap and use the edges of the plastic wrap to bring the dough together into a mass. Wrap and chill in the refrigerator for at least 1 hour and up to 3 days.
3. Make the caramelized onions: Heat the olive oil and butter in a cast-iron skillet or other large skillet over low heat. Spread the sliced onions in an even layer in the pan and let cook slowly, stirring only occasionally, for at least 40 minutes and up to an hour, or until onions turn soft, brown, and jammy. When they are nice and dark, add the balsamic vinegar and cook a little longer, scraping up the fond as it’s loosened by the vinegar. Remove from the pan and set aside. (These can also be made well in advance.)
4. On the day of: Plug the base pan, heating element and ceramic baking stone attachment into your Smart Oven+ and preheat to 375 degrees.  (For an oven without a baking stone attachment, preheat to 425 degrees.)  Let the pie dough thaw at room temperature for 5-10 minutes.  
5. Meanwhile, slice the potatoes into thin slices, about 1/8-inch thick (I like a mandolin a lot for this, but be careful!) and toss with olive oil, garlic, rosemary, about ½ teaspoon sea salt, and ¼ teaspoon black pepper. Set aside.
6. On a generously floured surface, roll the dough to 1/4- to 1/8-inch in thickness and about 12 to 14 inches in diameter. Re-flour the surface and the crust, as needed.
7. Sprinkle the grated cheese evenly over the crust, leaving about a 1- to 2-inch border at the edges, followed by the caramelized onions.  Place the potatoes on top in an even layer, and finish with another sprinkling of rosemary, salt, and Parmesan, if desired.
8. Fold the border up over the filling all the way around, and if desired, brush with heavy cream or egg wash.
9. Bake at 375 degrees for 45-55 minutes, or until the crust is deep golden-brown.