Heat the olive oil in a large skillet over medium heat. Add the onion and cook until softened and translucent, about 6 minutes. Add the garlic and cook 30 seconds, just until fragrant.
Add the ground pork and ground beef. Break the meat up with a spoon and cook until no longer pink, about 8 minutes. If there is a lot of grease, spoon off excess so the filling tastes rich, not oily.
Season with the salt, pepper, thyme, sage, cinnamon, clove, and nutmeg. Stir for 30 seconds to toast the spices in the fat (this is where the "holiday" flavor really shows up).
Sprinkle in the flour and stir for 1 minute. Pour in the broth and scrape up any browned bits from the bottom of the pan. Stir in the grated potato. Simmer, stirring often, until the liquid thickens and the potato is tender, about 10 minutes.
Turn off the heat and stir in the parsley. Spread the filling onto a plate or shallow pan so it cools faster, then refrigerate for 15 minutes. Cooling is the not-so-secret trick to a crisp bottom crust: hot filling melts butter before it hits the oven.