01 -  Preheat oven to 400°F. Line two baking sheets with parchment paper. 
 02 -  In a medium saucepan over medium heat, combine milk, water, butter, salt, and pepper. Bring to a gentle boil. 
 03 -  Remove the saucepan from heat and add flour all at once. Stir vigorously with a wooden spoon until a smooth ball forms and pulls away from the sides. 
 04 -  Return saucepan to medium heat and cook, stirring constantly, for 1 to 2 minutes to dry the dough slightly. 
 05 -  Transfer dough to a mixing bowl. Allow to cool for 3 minutes. 
 06 -  Beat in eggs one at a time, ensuring each is fully incorporated before adding the next. Dough should be glossy and pipeable. 
 07 -  Stir in Gruyère, Parmesan, chopped pickles, dill, and Dijon mustard until evenly distributed. 
 08 -  Transfer dough to a piping bag fitted with a large round tip or use a spoon. Pipe or drop tablespoon-sized mounds onto prepared baking sheets, spacing about 2 inches apart. 
 09 -  Bake for 20 to 25 minutes, until puffs are golden brown, crisp, and well-puffed. Do not open the oven during baking. 
 10 -  Remove from oven and let cool slightly before serving.