Finely chop the spring onion and half the watercress and place in a large bowl. Add the mince, 100g of the Parmigiano Reggiano, lemon zest and a good pinch of salt and plenty of ground black pepper. Use your hands to mix together.
Now divide and roll the mixture into about 24 balls, rolling them between the palms of your hands to make neat rounds.
Heat the oil in a non-stick frying pan, add the meatballs and fry for 8-10 minutes, turning occasionally until brown on all sides. Use a slotted spoon to transfer the meatballs to a plate.
Add the onion to the pan and sauté for 3 minutes until soft. Add the tomatoes and wine and sizzle for about 2 minutes. Add the purée and 100ml water, then simmer the sauce until the tomatoes have sof-tened down.
Add the meatballs to the sauce and simmer for 5 minutes. Roughly chop the remaining watercress, stir into the sauce and remove from the heat. Serve with plenty of Parmigiano Reggiano sprinkled over.