For the meatballs: Combine the breadcrumbs and milk in a large bowl and let sit for 5 minutes. Add the pork, Parmesan, salt, fennel seeds, garlic, onion, paprika, crushed red pepper, egg, parsley and a few turns of black pepper and mix to combine. It's easiest to do this with your hands. Form into 16 balls, each about 1 1/2 inches in diameter.
Heat a thin layer of oil in a skillet over medium-high heat and cook the meatballs in batches, turning occasionally, until browned on all sides, about 8 minutes. Set aside to cool slightly.
For the biscuits: Preheat the oven to 375 degrees F.
Flatten each biscuit with your palm, add a half piece of mozzarella and top with a meatball. Pinch the edges of the dough around each meatball to seal. Place seam-side down in a 9-inch square baking dish. Bake until lightly browned, about 30 minutes.
Meanwhile, heat the butter and oil in a small saucepan over medium-low heat. Add the garlic and crushed red pepper and cook for 2 minutes.
When the biscuits have baked for 30 minutes, brush them with the butter mixture, sprinkle with the parsley, Parmesan and some flaky salt and continue to bake until golden and cooked through, 5 to 10 more minutes. Let cool slightly and serve with warm marinara for dipping.