Steps
- Preheat the oven to 425°F / 220°C. Line baking sheets with parchment paper. Grate 250g of Gruyère and reserve one-fifth for topping.
- Bring 250ml of water, 80g of unsalted butter, and ¾ teaspoon of salt to a boil in a saucepan, stirring until the butter melts.
- Remove from the heat. Stir in 150g of all-purpose flour, 1 pinch of nutmeg, and 1 pinch of black pepper until combined.
- Return to low heat and stir for 1 minute. Remove and cool for 2 minutes.
- Beat in 4 large eggs, one at a time, mixing thoroughly after each. Stir in the rest of the cheese.
- Drop 1-tablespoon mounds onto the baking sheets, leaving room to expand. Brush with 1 beaten egg yolk and sprinkle with the reserved cheese.
- Bake for 25 minutes, until puffed and golden. Cool for 15 minutes before serving.