Photo: Courtesy of Joy The Baker.
Place racks in the center and upper third of the oven and preheat oven to 450 degrees F. Line two baking sheets with parchment paper, and set aside.
In a medium saucepan, combine water, butter, salt, pepper, and mustard. Bring to a simmer over medium heat. Once the butter is melted and the mixture is brought to a simmer, lower heat slightly, and add flour all at once. Stir immediately using a stiff heat-proof spatula or a wooden spoon. Stir until the mixture thickens, pulls away from the side of the pan, forms a mass, and the bottom of the pan is clean. The mixture will be glossy and damp.
We’re going to cook the dough over a low flame. This will allow moisture to evaporate from the dough, allowing it to absorb more fat when the eggs are added. Stir for about fives minutes. A thing gluten coat will form along the bottom and sides of the pan. When enough moisture has evaporated, the dough will steam and the cooking flour will smell slightly nutty.
Immediately transfer the dough to the bowl of an electric stand mixer fitted with a paddle attachment. Beat on medium speed for about one minute, releasing some of the heat from the cooked dough. Add four eggs, one at a time, beating until each egg is thoroughly incorporated. Stop the mixer between each egg addition and scrape down the bowl. After beating in the last egg the mixture should be glossy and thick — but still juuuust pourable. When scooped into a spoon, the dough should slowly pour off the spoon. If it doesn’t fall, or it fall in one giant clump… beat in the fifth egg. Whenever I make this recipe, I always need to add the fifth egg.
Using a spatula, fold in the chives, fresh herbs, lemon zest, and cheese until thoroughly combined.