Put the turkey mince into a bowl and add the grated onion, 1 of the grated garlic cloves, the fennel seeds, chilli flakes and lots of seasoning, and mix really well. Form into 16 balls.
Heat the olive oil in a large non-stick frying pan and fry the meatballs, in batches, for 3-4 minutes or until golden all over. Scoop out onto a plate.
Add the finely chopped onion, remaining garlic clove and a generous splash of water, and cook for 5-10 minutes or until the onion has softened. Tip in the chopped tomatoes and simmer for 15 minutes, then use a hand blender to whizz until smooth. Return the meatballs and simmer for 10 minutes until cooked through.
Cook the spaghetti following pack instructions in a large pan of boiling salted water, then drain, reserving some of the cooking liquid.
Tip the spaghetti into the pan, toss with the sauce and meatballs, and divide between plates. Scatter over the basil and parmesan before serving.