Preheat the broiler/grill to high (or as high as it goes without smoking the kitchen out).
In a large mixing bowl, use a fork to mix the bread, milk, grated onion and Worcestershire until combined. Leave to sit for a few mins for the bread to fully absorb the moisture. Mix in the parsley, parmesan, garlic and seasoning until combined, then mix in the egg until beaten.
Add the pork and beef and use your hands to mix until everything is evenly dispersed. Don't squeeze the mixture; rather, use your fingers to gently mix it. Whilst it's important that everything is properly combined, it's more important that you don't overwork the meat and squeeze it into oblivion, otherwise the meatballs will come out tough and dry.
Line a large baking tray with baking/parchment paper, then use a 1 tbsp measuring spoon to scoop out chunks of the mixture and spread them out on the tray (you should get 24 chunks). One by one, roll the chunks of meat into meatballs, then spray with oil (consider wetting your hands if they're sticking).
Place the meatballs under the grill/broiler and cook until browned all over (I typically turn them once halfway). You're not looking to cook them all the way through at this point, although naturally they probably will. This is just to brown the outside and develop some flavour. Depending on how hot your grill/broiler is and how close you can get the meatballs, it'll likely take around 15 minutes (just be vigilant though, as timings may vary).