Preheat your oven to 425°F (220°C). Line two baking sheets with parchment paper.
In a medium saucepan, combine the water, butter, and salt. Bring to a boil over medium heat.
Once the butter is melted and the mixture is boiling, add the flour all at once. Stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan. Continue stirring for 1-2 minutes to cook off some of the moisture.
Remove the pan from the heat and let it cool for a few minutes. Add the eggs, one at a time, beating well after each addition until the dough is smooth and glossy. The dough will appear to separate but keep stirring until it comes back together.
Stir in 2 cups of the grated Gruyère cheese, black pepper, nutmeg (if using), and Dijon mustard (if using).
Using a cookie scoop, drop tablespoon-sized mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Lightly spray the tops of the gougères with cooking spray and sprinkle a small amount of the extra grated Gruyère cheese on top of each one.
Bake in the preheated oven for 10 minutes. Reduce the oven temperature to 375°F (190°C) and continue baking for an additional 15-20 minutes, or until the gougères are puffed and golden brown.
Turn off the oven and prop the oven door open with a wooden spoon. Let the gougères sit in the oven for 5-10 minutes to dry out the insides.
Serve the gougères warm or at room temperature.