1. Fry the onion in the oil for 5 mins, stir in the mince, then fry for a further 5 mins until evenly, coloured. Mix in the jar of sauce. Bring to the boil, cover, then simmer for 20 mins, adding a splash of water if it starts to get too thick.
2. Heat grill to high. Cut the polenta into about 20 slices. Tip the mince into a shallow ovenproof dish, then arrange the polenta on top in overlapping slices. Sprinkle with the Parmesan. Grill for about 5 mins until the polenta and cheese are tinged with gold.