While these are fabulous reheated the next day, they’re so, so good warm right out of the oven. Since I still have some rosemary leftover from Danielle’s garden, I chopped it finely and tossed it in at the end along with a healthy dollop of good Dijon mustard. Feel free to use cheddar if you’d prefer, and add any mix of finely chopped herbs you’d like. These also freeze beautifully if you have leftovers; you can even freeze the mounds of dough before you bake them and bake them off right from the freezer (good party trick).
Position two racks into bottom third of the oven and preheat to 425 degrees F. Line two baking sheets with silicone baking mats or parchment paper (or just oil well with olive oil).
Bring the milk, water, butter, and salt to a vigorous boil in a heavy-bottomed small to medium saucepan over high heat. Whisk until the butter melts completely. Add the flour all at once, lower the heat to medium-low, and stir quickly with a wooden spoon until the dough comes together in a ball. Keep stirring until dough is no longer sticky and is, instead, smooth, 1-2 minutes.
Transfer the dough into the bowl of a mixer fitted with the paddle attachment. Let the dough sit for a minute, then add the eggs one at a time and beat until the dough is nice and shiny. At the very end, stir in the grated cheese, mustard, chopped rosemary, and pepper.
Drop rounded tablespoons of dough onto each pan with about 2 inches of space separating each one. Slide the baking sheets into the oven and immediately turn the oven temperature down to 375 degrees F. Bake for 15 minutes, then rotate the pans from front to back and top to bottom. Continue baking until the gougères are golden, firm, and puffed, another 12 to 15 minutes. Serve warm, or transfer the pans to racks to cool.