Preheat your oven to 220°C (200°C fan-forced). Line a baking tray with baking paper.
Combine the water, milk, butter, and salt in a saucepan over medium heat. Once the butter has melted, reduce the heat to low and add the flour all at once. Stir vigorously until the flour is fully incorporated and a thick dough forms. Continue pressing the dough against the pan for 3 minutes to cook the flour and remove excess moisture.
Remove the pan from the heat and let the dough cool for about 5 minutes. Add the eggs, one at a time, stirring well after each addition until fully incorporated.
Fold in the grated Gruyère (or Cheddar). Using a tablespoon or small cookie scoop, drop small mounds of dough onto the prepared baking sheet, spacing them out enough to rise. Sprinkle the tops with grated Parmesan.
Bake at 200°C / 180°C fan-forced for 25 minutes, or until puffed, deep-golden and crisp on the outside and sides. Avoid opening the oven in the first 20 minutes or they will collapse. Serve warm and Bon App!
