Preheat the Oven:

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

Prepare the Choux Pastry:

In a medium saucepan, combine water, milk, butter, and salt. Bring to a boil over medium heat, ensuring the butter completely melts.
Add the flour all at once, stirring vigorously with a wooden spoon. Keep stirring until the mixture forms a dough that pulls away from the sides of the pan.
Cook for another 1-2 minutes to dry out the dough slightly.

Add Eggs:

Transfer the dough to a mixing bowl and let it cool for a few minutes. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. The dough should be smooth and shiny.

Add Cheese and Seasonings:

Stir in the grated cheese, mustard (if using), and a pinch of cayenne pepper or nutmeg. Season with freshly ground black pepper.

Pipe or Spoon the Dough:

Using a piping bag or two spoons, form small mounds of dough on the prepared baking sheet, spaced about 2 inches apart. Sprinkle additional cheese on top.

Bake:

Bake in the preheated oven for about 20-25 minutes, or until the gougères are puffed and golden brown. They should be crisp on the outside and soft on the inside.

Serve:

Serve warm or at room temperature as an appetizer or a snack.