Add chicken thighs to a food processor and pulse in short bursts until it resembles ground meat, scraping down the sides with a spatula as needed. Scoop out into a large mixing bowl and add breadcrumbs, parmesan, parsley, onion, garlic, Worcestershire sauce, egg, salt & pepper. Use your hands to combine until everything is evenly distributed (don't mix it into oblivion or it'll come out tough/dry).
Oil a wire rack. I find this easiest with paper towels. 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 finely diced onion to an 8x12" baking dish and combine with a drizzle of oil. Place in the oven at 200C/390F for 8-10mins, or until soft & lightly golden. Add in garlic and allow it to sizzle for a minute or so, then pour in passata. Swill out the jar with ~1/4cup water and add that in too, then pour in chicken stock. Add tomato puree (paste), oregano, sugar, salt, pepper & chilli flakes if you're using them. Whisk to combine then stir in 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 cover to one side, then give the orzo a good stir, scraping up any orzo stuck to the dish as needed.
Place back in the oven for another 5-10mins, or until the orzo is al dente. It should still be fairly saucy, with the orzo 90% cooked through. Stir through basil and check for seasoning, then toss in the meatballs.
Serve up with plenty of parmesan and any leftover parsley/basil you've got!