Preheat the oven to 180C/gas 4. Cut the fennel into 2–3cm wedges and lay out in a roasting tray. Add a splash of oil, season, then roast in the oven for 30 minutes.
Sprinkle over the pine nuts and pumpkin seeds and pop back in the oven for 8 minutes, until the fennel is cooked through. Remove from the oven and leave to cool for 5 minutes.
Place 2 garlic cloves and the chilli in a food processor and blitz until finely chopped. Add the fennel, pine nuts and pumpkin seeds, then pulse again until chopped but still quite chunky.
Grate in the parmesan, then add the breadcrumbs, egg yolk and a pinch of salt and pepper. Pulse until everything just comes together.
Drizzle a roasting tray with olive oil. With wet hands, roll the mixture into about 20 walnut-sized balls, then place on the tray and leave in the fridge for 15 minutes. Preheat your oven again to 200C/gas 6.
Drizzle the polpette with oil and bake for 20–25 minutes, until the balls are golden and crisp.
While they are cooking, make a tomato sauce. Place a medium-sized, non-stick pan over a medium heat and pour in a glug of oil. Finely slice the remaining garlic clove and add to the pan, along with the parsley stalks. Fry for 1 minute before pouring in the tinned tomatoes.
Fill the tin halfway with water and add, along with the balsamic, then gently bring to the boil. Reduce to a simmer and cook over a low heat for 8–10 minutes, until thickened. Pour the sauce into a food processor and blitz until smooth, then return to the pan and keep warm until needed.
Divide the sauce between plates, arrange the polpette on top, sprinkle with the chopped parsley and serve.