Preheat the oven to 425°F. Add the ground beef, mild Italian sausage, breadcrumbs, Parmesan, egg, milk, garlic, salt, and pepper to a large bowl. Gently mix until just combined, being careful not to overwork the meat
Divide the meat mixture into 12 equal portions, about 3 tablespoons each, and roll each portion into a ball.
Place the meatballs on a parchment-lined baking sheet and bake at 425°F for 8 minutes. The meatballs will not be fully cooked at this point since they will finish cooking inside the crescent roll cups.
Remove the meatballs from the oven and immediately reduce the oven temperature to 375°F. Set the meatballs aside while you prepare the crescent dough.
Unroll the crescent roll dough and pinch the perforated seams together to form one large sheet. Gently press or roll the dough into an even rectangle and cut it into 12 roughly equal squares
Lightly grease a 12-cup metal muffin pan. Press one piece of crescent dough into each muffin cup, letting the dough come slightly up the sides. Be careful not to stretch the dough too thin on the bottom
Sprinkle a small amount of mozzarella cheese into the bottom of each crescent cup. Place one partially baked meatball into each cup and spoon about 1 heaping teaspoon of marinara sauce over each meatball
Top each cup with additional shredded mozzarella and a sprinkle of Parmesan cheese.
Bake at 375°F for 13–15 minutes, or until the crescent dough is golden brown, the cheese is melted, and the meatballs are completely cooked through. Let the meatball cups cool in the muffin pan for about 5 minutes before carefully removing them. Serve warm with extra marinara sauce for dipping.