Cheesy Meatball Sub Puff Pastry Pockets

1. **Prepare the Meatballs:** In a large bowl, combine the ground beef, ground pork, milk or sour cream, minced garlic, large eggs, Parmesan cheese, chopped parsley, chopped basil, finely chopped shallot, onion powder, garlic powder, Italian seasoning, black pepper, red chili flakes, a pinch of salt, and panko breadcrumbs. Mix until just combined, being careful not to overmix.
2. Form the mixture into meatballs of your desired size. Place them on a baking sheet and freeze for 1 hour to help them hold their shape.
3. Preheat oven to 425°F (220°C). Cook the frozen meatballs at 425°F for 20 minutes, then broil for 3 minutes until browned and cooked through. Remove and let cool slightly.
4. **Assemble the Pockets:** Preheat oven to 400°F (200°C). Roll out the thawed puff pastry sheets. Cut each sheet into squares or rectangles, depending on your desired pocket size. 
5. Place a cooked meatball or two, and a slice of mozzarella cheese on one half of each puff pastry piece. Fold the other half over to create a pocket and crimp the edges with a fork to seal.
6. **Prepare Egg Wash:** In a small bowl, whisk together 1 large egg with 1 teaspoon of milk or water.
7. Brush the tops of the assembled puff pastry pockets with the egg wash and sprinkle with coarse salt.
8. **Bake:** Bake at 400°F (200°C) for 20 minutes, or until the pastry is golden brown and puffed.