2In a bowl, combine ground beef, ground pork, soaked panko, Parmigiano, egg, 4 minced garlic cloves, salt, pepper, and red pepper flakes. Mix gently with your hands until just combined—don't overmix.
3Roll mixture into 1.5-inch balls; you'll get about 16 meatballs.
4Heat 2 tbsp olive oil in a large skillet over medium-high heat. Working in batches, brown meatballs on all sides (about 8 minutes total per batch), then transfer to a plate.
5In the same skillet, heat remaining 1 tbsp olive oil. Add 3 minced garlic cloves and cook until fragrant, about 30 seconds.
6Stir in tomato paste and cook for 1 minute. Add crushed tomatoes, basil, and oregano. Bring to a simmer.
7Return meatballs to the skillet and gently stir to coat. Simmer uncovered for 15–20 minutes, stirring occasionally, until sauce thickens slightly and flavors meld.
8Split sub rolls and arrange on a baking sheet. Broil for 1–2 minutes until lightly toasted.
9Divide meatballs and sauce among the roll bottoms. Top each with fresh mozzarella slices.
10Broil for 2–3 minutes until cheese melts and bubbles. Scatter fresh basil on top.