Stir panko and milk together in a large bowl; rest 5 minutes.
Add beef, pork, Parmesan, egg, garlic, parsley, Italian seasoning, salt, pepper, and red pepper flakes. Mix gently with hands just until combined.
Preheat oven to 400°F. Line a baking sheet with parchment.
Portion into 12 balls (about 1/4 cup each) and roll into 2-inch rounds.
Bake 18–20 minutes until browned and 160°F in the center.
Warm marinara in a large skillet over medium-low. Add meatballs, cover, and simmer 15 minutes, turning once.
Serve over pasta or in rolls with extra Parmesan and parsley.