Preheat the oven to 450˚F (230˚C). Arrange racks in lower and upper thirds of the oven. Line 2 baking sheets with parchment papers.

In a medium saucepan, add the water, butter, salt and mustard. Bring to a boil over high heat.

Once butter has melted, remove the pan from heat. Add the flour all at once and stir vigorously with a wooden spoon until the mixture comes together and pulls away from sides of pan, about 2 minutes. Return the pan to medium-low heat. Cook and continue to stir vigorously, until a dry film forms on bottom and sides of pan, about 3 – 4 minutes. The dough is ready when it has a shine.

In a large mixing bowl with an electric mixer fitted with a paddle attachment, add the hot dough. Beat the dough on a medium-low speed for 2 minutes until it is just warm to touch. With the mixer running, add the beaten egg in four additions. Make sure the egg has been absorbed and the dough is smooth out before adding the next. Scrape down the sides of the bowl after each addition. The dough should be smooth and creamy at the end. Mix in cheese.

Transfer the dough into a piping bag fitted with a ½-inch round tip. Pipe about 1 tablespoon dough on the prepared baking sheets, spacing them at least 1-inch apart. To smooth out the sharp tops, push down slightly with a wet finger.

Bake in the oven for 5 minutes. Then turn the heat down to 350˚F (180˚C). Bake for another 20 -25 minutes, rotating the pans once during baking for even cooking. The gougères should be puffed and golden. They should be dry in the center and sound hollow when tapped. Serve warm or at room temperature.