Place the cured ham and sage on top of the steaks, secure with toothpicks. Heat the oil in a frying pan. Fry the saltimbocca in batches on the ham side for approx. 1 min., season, turn, fry for a further 2 mins., remove and keep warm.
Heat a little oil in the same pan. Fry the courgette and mushrooms in batches for approx. 2 mins. on each side, season, remove and keep warm.
Pour the prosecco into the pan, reduce to approx. 2 tbsp. Turn down the heat, then remove the pan from the hob. Add the butter gradually while stirring with a whisk. Briefly return the pan to the hob to gently warm the sauce.