In a large bowl, combine the meatball ingredients and mix thoroughly to combine. Let the mixture chill while your sauce is cooking.
While the meat mixture is chilling, make the sauce: preheat the oven to 350F.
Heat a large dutch oven or a heavy oven-safe pot over medium heat.
Once the pot is hot, add the olive oil and then onion. Season with a pinch of sea salt and sauté until onion is golden, about 15 minutes.
Turn the heat down to medium-low and add the garlic and chilli flakes. Sauté a further 5 minutes.
Add the tomato passata and chicken broth. Stir to combine.
Cover and place in the oven for 1 hour.
After one hour, remove from the oven and stir the sauce.
Remove the meatball mixture from the fridge.
Take about 2 tablespoons of the meat mixture, roll it into a ball and drop into the tomato sauce. Repeat with the remaining meat mixture.
Gently stir the sauce to coat the meatballs, cover and place back in the oven for 1 hour.
Remove from the oven, season with salt and pepper to taste.
Serve topped with parmesan and basil leaves.