1.  Prepare the Bread: In a small bowl, soak the stale bread in the milk for 5-10 minutes until soft. Then, squeeze out all the excess liquid with your hands.

2.  Combine Meatball Mixture: In a large bowl, add the ground meat, the soaked bread, egg, grated Parmesan, parsley, and minced garlic. Season generously with salt and pepper.

3.  Form the Meatballs: Gently mix everything with your hands until just combined. Do not overmix. Roll the mixture into evenly-sized golf ball meatballs.

4.  Cook the Meatballs: Heat the olive oil in a large skillet over medium heat. Pan-fry the meatballs in batches for 8-10 minutes, turning them to get a golden-brown crust on all sides. Remove them from the skillet and set aside.

5.  Make the Sauce: In the same skillet, sauté the chopped onion until soft and translucent. Pour in the tomato passata. Swirl a little water in the empty passata jar and add it to the pan. Season with salt and simmer for 10 minutes.

6.  Simmer Together: Gently place the seared meatballs back into the skillet with the sauce. Let them simmer together for 5 more minutes to heat through and absorb the flavors.

7.  Finish and Serve: Turn off the heat and stir in the fresh basil leaves. Serve immediately over pasta or with crusty bread.