Pre-heat the oven to 400°F. Line a baking sheet with parchment paper.

Bring the milk, water, and butter to a boil in a 3-quart saucepan. Lower the heat and add the flour and salt. Stir vigorously with a wooden spoon until the dough is dry and smooth in the pan. The temperature should read 165 – 175˚F on an instant read thermometer. Let the mixture cool for 5 minutes (if you want to check, it should be lower than 145˚F).

Using an electric hand mixer, add the eggs to the pan, one at a time and beating to incorporate. Then add the egg white and continue to beat until the dough comes together and is smooth. Add the Dijon mustard, pinch of nutmeg and the Gruyère cheese. Stir by hand until combined.

Using a small scoop or spoon, drop 1½ tablespoons of dough, 2-inches apart, onto the prepared cookie sheets. Alternatively, pipe drops of the batter onto the cookie sheet using a pastry bag. Sprinkle a little Parmesan cheese on top of each Gougère.

Bake at 400°F for 15 minutes, and then lower the temperature of the oven to 350°F and bake 5 to 10 minutes longer until the Gougères are golden brown and puffed up. Do not open the oven door while they bake.

Remove from the oven and transfer to a cooling rack. Pierce the Gougères with the tip of a sharp knife to release the steam. (This helps keep the Gougères’ texture so that the interior moisture doesn’t cause them to get soggy.)

Serve warm the same day. If making ahead, you can reheat the Gougères at 300°F for about 15 minutes. To make days ahead, freeze the Gougères unbaked, and bake the day you are planning on serving them. Let the frozen dough balls thaw on the counter for about 30 minutes before baking.