Heat the oven to 200C/180C fan/gas 6. Put the chicken in a food processor and pulse until finely chopped. Scrape into a bowl and add the breadcrumbs, egg, onion, parmesan, half the crushed garlic, and half the oregano. Season really well, and mix with clean hands until everything is evenly distributed. Shape into 20 meatballs, and put onto an oiled baking tray. Brush the meatballs with 1½ tbsp oil. Bake for 20 minutes until cooked through and golden brown.
Meanwhile, heat the remaining ½ tbsp oil in a deep frying pan. Fry the remaining garlic for 30 secs, then add the chopped tomatoes and reserved oregano. Season with salt, pepper, and a pinch of sugar. Simmer for 10-15 mins stirring frequently, until the tomatoes break down and you get a thick sauce.
Bring a large pot of salted water to a boil. Add the spaghetti and cook for 10 mins until al dente.
Add the cooked meatballs to the tomato sauce and gently stir through. Heat for a few minutes while you drain the spaghetti and spoon into warmed pasta bowls. Serve the meatballs and sauce on top. Drizzle over a little olive oil and a grating of more parmesan, if you wish.