Heat the oil in a non-stick frying pan, sauté the onion and garlic for approx. 2 mins., gradually add the spinach, cook until all the liquid has evaporated, season. Allow the spinach to cool a little, chop finely, stir in the parmesan.
Season the veal steaks, top with the ham. Spread 2 tbsp of filling on top, roll up the steaks and secure each one with a toothpick. Over a medium heat, fry the involtini in hot oil on all sides for approx. 4 mins. per batch.
Heat the oil in a pan. Sauté the onion and garlic, add the tomatoes and water, season. Simmer the sauce over a medium heat for approx. 20 mins., stirring occasionally.
Spoon a little sauce into the prepared dish, add the involtini, spread the remaining sauce around the involtini, sprinkle with cheese.
Bake for approx. 20 mins. in the centre of an oven preheated to 180°C. Garnish with basil.