The evening before you plan to serve, combine the bread flour, water, active sourdough starter, and salt in a large bowl. Mix until no dry flour remains. The dough will be very sticky. This is expected with a high hydration focaccia dough.
Cover the bowl and let the dough rest for 30 minutes at room temperature.
With wet hands, perform one round of stretch and folds. Grab one edge of the dough, stretch it up, and fold it over to the opposite side. Rotate the bowl 90 degrees and repeat four times total around the bowl. This builds gluten structure without kneading.
After the stretch and fold, perform a slap and fold by turning the dough out onto a clean unfloured surface and slapping it down, then folding it over itself repeatedly until the dough feels slightly smoother and holds its shape. Tuck the seam under the bottom to create surface tension.
Place the dough seam side down back into the bowl or a lidded proofing container. Cover tightly and leave at room temperature to ferment overnight for 8 to 12 hours depending on the temperature of your kitchen. In a warmer kitchen around 75 to 78 degrees F, check around 8 hours. In a cooler kitchen, allow the full 12 hours. The dough is ready when it has doubled or tripled and feels airy and jiggly.
When ready to assemble, prepare the garlic butter coating by melting the butter and oil together in a small bowl. Stir in the minced garlic and dried parsley. Set aside.
Prepare the Parmesan cornstarch coating by combining the finely grated Parmesan, cornstarch, and dried parsley in a shallow bowl. For the finest texture, cut a block of Parmesan into cubes, process in a food processor until it resembles coarse sand, then add the cornstarch and pulse briefly. Pre-grated refrigerated Parmesan also works well.
Line a 9x13 inch baking pan with parchment paper. Working quickly, tear off a golf ball sized piece of dough. Dip it in the garlic butter mixture, coating all sides, then roll it in the Parmesan cornstarch mixture until evenly coated. Place in the prepared pan. Repeat until all 25 balls are coated and arranged in five rows of four with a small amount of space between each ball. Cover the pan with a lid or plastic wrap and allow the dough balls to proof at room temperature for approximately 2 hours. They should puff noticeably and expand toward each other. While the dough is proofing, bake your meatballs if you have not already done so and allow them to cool completely.
Once the dough has proofed and the meatballs are cooled, press one meatball firmly into the center of each focaccia ball. Push it down so the dough comes up around the sides but the top of the meatball remains visible and exposed. Add approximately half a teaspoon of marinara sauce on top of each meatball, spooning it directly into the well where the dough meets the meat.
Preheat your oven to 425 degrees F.
Bake the assembled pan for 20 minutes until the dough is set and beginning to turn golden. Remove the pan from the oven and add a generous pinch of shredded mozzarella on top of each ball directly over the meatball and sauce.
Return the pan to the oven for an additional 10 to 15 minutes until the mozzarella is fully melted and bubbling and the tops of the dough balls are deep golden. If the cheese begins to darken faster than you would like, loosely tent the pan with foil for the final few minutes.
If you prefer to skip the cheese, bake for a total of 30 to 35 minutes in a single stretch without removing the pan.
Allow the pan to cool for 10 to 15 minutes before serving. The meatballs retain significant heat so give it a few minutes before tearing in.