Instructions

1. Boil the water, butter, and salt. Place the water, butter, salt, and mustard if using in a 2- to 4-quart saucepan and bring to a rolling boil over medium-high heat, stirring to melt the butter.
2. Add the flour. Remove the pan from heat and add the flour all at once. Stir vigorously with a wooden spoon until the mixture comes together and resembles mashed potatoes.
3. Cook the dough. Return the pan to medium-low heat and stir for 3 to 5 minutes to dry out the dough. The dough is ready when it smells nutty, glistens, and is thick enough to hold a spoon upright. A film of starch on the bottom of the pan is normal.
4. Cool the dough. Transfer the dough to the bowl of a stand mixer fitted with a paddle attachment. (Alternatively, use an electric hand mixer or beat by hand with a stiff spatula.) Beat the dough on medium-low speed until it stops steaming and is just warm to the touch, about 1 minute.
5. Add the eggs and cheese. Continue beating and add the eggs in 4 additions. Wait for each addition to be absorbed and for the dough to smooth out before adding the next. Scrape down the sides of the bowl if needed between additions. In the end, the dough should come together in a very smooth, creamy batter. Beat in the cheese.
6. Freeze the gougères. For make-ahead gougères, scoop rounded tablespoons of dough onto baking sheets lined with silicone mats or parchment paper. Place them as close together as possible without touching. Cover loosely with plastic wrap. Freeze until solid, then transfer to an airtight container or a ziptop bag for long-term storage.
7. Bake the gougères at high heat. Arrange 2 racks to divide the oven into thirds and heat to 450°F. Place the frozen gougères at least 1 inch apart on a parchment paper-lined baking sheet. Bake for 5 minutes.
8. Reduce the oven temperature. Reduce the heat to 350°F. Bake until puffed, deep golden-brown, and dry to the touch (the cheese may still be bubbling a bit), rotating the sheets between racks and from front to back halfway through baking, 20 to 25 minutes more. The finished gougères will feel light and hollow when picked up.
9. Cool. Transfer the baking sheets to a cooling rack. Serve warm or at room temperature.