Prepare Meatball Mixture: In a large bowl, combine the minced beef and pork, egg, milk, oregano, basil, Italian pasta mix, Italian herb mix, fresh rosemary, and grated Grana Padano cheese.
Mix Gently: Using hands or a wooden spoon, gently mix until just incorporated. Ensure the mixture remains moist and cohesive, with visible pieces of ground meat.
Form Meatballs: With lightly oiled hands, form the mixture into meatballs. This prevents sticking and helps incorporate a bit more moisture into the meatballs.
Brown Meatballs: Heat oil in a skillet over medium heat. Brown the meatballs on all sides to develop a golden crust, then set aside on a baking tray. This may require doing in batches.
Prepare the Sauce: In the same pan, add additional oil if needed, and soften the chopped onion and minced garlic. Stir in the tomato puree, then add the passata. Season with salt and pepper.
Cook Meatballs in Sauce: Add the meatballs back to the pan with the sauce. Simmer for about 10 minutes, or until the meatballs are cooked through.