Sift the flour and a little salt into a bowl. Set a non-stick pan on the heat with 150ml cold water and the chopped butter. Bring to the boil, stirring until the butter melts. Tip in all the flour and, using a wooden spoon, beat hard until the mix becomes smooth and starts to leave the side of the pan.
Remove from the heat, beat in the gruyère and half the Parmesan and cool for 5 mins. Gradually beat in the eggs until you have a smooth, thick, but spoonable mix – you may not need all the egg. Can be set aside now for 2 hrs without chilling.
When ready to serve, heat the oil in a deep-fat frying pan to around 180C. Using 2 teaspoons scoop up neat dollops and carefully drop into the hot oil. Fry 6-8 dollops a time for 3-4 mins until golden and crisp – dunk the dollops under to brown evenly. Reheat the oil as necessary. Drain on paper towels, sprinkle with the rest of the Parmesan. Serve hot.