Add half of the chopped onion into a large mixing bowl along with bread crumbs, cheese, oregano, pepper and 1 teaspoon of the salt. Mix with a fork until well combined.
Add the milk and mix well. Add the ground meat and egg. Knead the mixture by hand until all of the ingredients are evenly distributed – set aside.
Add the olive oil and the rest of the onions, carrot and celery to the cooker and sauté. Pour in the tomato puree, salt, and water and mix well.
Move the bowl with the meat mixture next to the cooker and start making meatballs. As you shape each meatball, drop them into the cooker in an even layer.
Close the lid and set the valve to the pressure position. Pressure cook on HIGH for 5 minutes.
When time is up, open the pressure cooker using the natural release method.
Gently scoop meatballs out onto freshly cooked spaghetti and smother in tomato sauce.