Preheat the oven to 400F/200C.
Add all of the meatball ingredients to a large mixing bowl and use your hands to combine until everything is evenly distributed (don't mix it into oblivion or it'll come out tough/dry).
Use paper towels and oil to grease a wire rack (helps prevent the meatballs from sticking). Use a 1 tbsp measuring spoon to scoop out 20 chunks of meat and place them on the wire rack. One by one, roll the chunks into meatballs. Liberally spray all over with oil.
Add the finely diced onion to an 8x12" baking dish (or similar size) and combine with a drizzle of oil. Place in the oven for 8-10mins, or until soft & lightly golden. Add in the garlic and allow it to sizzle for a minute or so (the excess heat will lightly cook it), then pour in the passata. Swill out the jar with ~1/4cup water and add that in too, then pour in chicken stock. Add the tomato puree (paste), oregano, salt, pepper & chilli flakes. Whisk to combine, then stir in the orzo.
Add the meatballs & wire rack on top, then carefully transfer everything to the oven. Bake for 20-25mins, or until the meatballs are golden brown and piping hot through the centre. Remove the meatballs and place to one side, then give the orzo a good stir, scraping up any orzo stuck to the dish as needed.
At this point, the orzo should be al dente with plenty of sauce. If you find it still needs to cook a little more, you can place it back in the oven for another 5-10 minutes. Keep in mind the orzo will continue cooking slightly and soak up more of the sauce as you toss in the meatballs and serve up. If the sauce over-thickens before the orzo is cooked, just add a splash more stock or hot water to help it cook through.
Stir through the basil and check for seasoning, then toss in the meatballs. Serve up with plenty of parmesan and any leftover parsley/basil you've got on hand.